One of the 'basics' of many karate styles is the down-block / punch combination often known as 'gedan barai, seiken tsuki'. It is taught right away to beginners in many dojos.
Typically, it involves the karateka crossing their own centerline with the blocking hand, then sweeping down in a blocking (or sweeping) motion, ending up somewhere in the area of the knee on the same side as the arm doing the blocking. This is then withdrawn to the hip as the other arm throws a straight punch to the opponent's body.
The first question that beginners ask is what it is for. It is usually taught in the beginning as a defense against a kick, followed by a counter-attack (the punch).
Some beginners (and a lot of outsiders) criticize this application and point out:
1) They are not very likely to be kicked by an assailant in the street.
2) Even if they were, they would not be likely to try to block a kick like that.
3) Crossing center takes time; by the time you draw your arm back to 'cross center', the kick has been delivered.
These comments have validity. However, the lack of a fuller understanding of the technique can then prevent the student from bothering to get a handle on the technique to the point of being able to apply it.
Consider, first of all, that most of the beginner's basic movements have reasons to practice them that go well beyond the technique itself. In fact, most of the 'upper body' basics get the beginner's body to move in ways that are useful. For gedan barai, this would include hip rotation. It is important to develop power through turning the body, and for gedan barai to be effective, much more than muscle power is required.
Please note that moving the arm from the chest (like one is thumping one's chest in a salute) to the opposite knee area does not involve major muscle groups. In other words, it's not a powerful move if delivered by muscle power alone. One can of course build strength specifically to perform this motion, but one can gain much more speed and power in the movement when the hips are involved. This is, I believe, one of the 'secrets' of gedan barai. One of the first explorations of power-generation via the hips.
Also, consider that the technique does not start when the block or sweep is delivered to the incoming kick. it starts when the karateka begins to move. Crossing the centerline of one's own body gives ample opportunity to intercept all manner of incoming attacks. One example would be a straight punch. Although the 'middle body block' is typically prescribed for such attacks, the 'low body block' works just as well, and gives different opportunities when applied. The punch comes in, the karateka intercepts with his forearm as he crosses his centerline, then reverses in a circular motion to complete the 'down block' (or sweep), which tosses the attacker's arm down and out away from the karateka's body. Because it is applied with force, using the hips, this can turn an attacker's body, causing unbalance and opening them up to the follow-on strike to the body (seiken tsuki).
That's one example. Same technique, but no kick was involved. Works just the same regardless. Why would it be preferred to a simple middle-body block? Well, it isn't always, but it can be useful when a smooth intercept is desired to prevent the attacker from realizing instantly that their incoming punch had been intercepted and then firing their other fist as an instant follow-up. A good sweeping motion can deflect the punch, turn the body, and open them up for your punch before their brains can realize that their punch was deflected away, because there is very little impact to them until they get punched at the end of the technique.
As well, the 'barai' can be an armbar, applied at shoulder level but then brought to the traditional low-block position using the exact same hip movement to generate power. The straight punch follow-up would then be turned into a face-punch delivered a tad low...where the opponent's face is now...
The 'barai' can be a release for a collar-grab as well; again, the movements are the same. Cross center line, minor circular motion, then twist the body and apply the 'low block', which in this case would not block a kick, but take a properly-twisted hand/wrist to knee level, where again a follow-up punch could be applied.
In summary, it is not the basic notion of a 'low block, punch' combination that makes gedan barai so useful. It is the basic movements that are developed when practicing this technique, which open up a world of techniques beyond the simple ones I've described here. The body learns to smoothly intercept by crossing over (seen again and again in many Okinawan kata in various sorts), to apply power from the hips to turn the opponent, open him to counter attack, and off-balance him. It is the movement that is most important, because it builds the power, speed, balance, and confidence necessary to apply this technique in a wide variety of ways.
Gedan Barai is much more than a simple low-block, punch. It is a mistake to dismiss it as a useless defense against an attack one would never see 'in the street'.
Typically, it involves the karateka crossing their own centerline with the blocking hand, then sweeping down in a blocking (or sweeping) motion, ending up somewhere in the area of the knee on the same side as the arm doing the blocking. This is then withdrawn to the hip as the other arm throws a straight punch to the opponent's body.
The first question that beginners ask is what it is for. It is usually taught in the beginning as a defense against a kick, followed by a counter-attack (the punch).
Some beginners (and a lot of outsiders) criticize this application and point out:
1) They are not very likely to be kicked by an assailant in the street.
2) Even if they were, they would not be likely to try to block a kick like that.
3) Crossing center takes time; by the time you draw your arm back to 'cross center', the kick has been delivered.
These comments have validity. However, the lack of a fuller understanding of the technique can then prevent the student from bothering to get a handle on the technique to the point of being able to apply it.
Consider, first of all, that most of the beginner's basic movements have reasons to practice them that go well beyond the technique itself. In fact, most of the 'upper body' basics get the beginner's body to move in ways that are useful. For gedan barai, this would include hip rotation. It is important to develop power through turning the body, and for gedan barai to be effective, much more than muscle power is required.
Please note that moving the arm from the chest (like one is thumping one's chest in a salute) to the opposite knee area does not involve major muscle groups. In other words, it's not a powerful move if delivered by muscle power alone. One can of course build strength specifically to perform this motion, but one can gain much more speed and power in the movement when the hips are involved. This is, I believe, one of the 'secrets' of gedan barai. One of the first explorations of power-generation via the hips.
Also, consider that the technique does not start when the block or sweep is delivered to the incoming kick. it starts when the karateka begins to move. Crossing the centerline of one's own body gives ample opportunity to intercept all manner of incoming attacks. One example would be a straight punch. Although the 'middle body block' is typically prescribed for such attacks, the 'low body block' works just as well, and gives different opportunities when applied. The punch comes in, the karateka intercepts with his forearm as he crosses his centerline, then reverses in a circular motion to complete the 'down block' (or sweep), which tosses the attacker's arm down and out away from the karateka's body. Because it is applied with force, using the hips, this can turn an attacker's body, causing unbalance and opening them up to the follow-on strike to the body (seiken tsuki).
That's one example. Same technique, but no kick was involved. Works just the same regardless. Why would it be preferred to a simple middle-body block? Well, it isn't always, but it can be useful when a smooth intercept is desired to prevent the attacker from realizing instantly that their incoming punch had been intercepted and then firing their other fist as an instant follow-up. A good sweeping motion can deflect the punch, turn the body, and open them up for your punch before their brains can realize that their punch was deflected away, because there is very little impact to them until they get punched at the end of the technique.
As well, the 'barai' can be an armbar, applied at shoulder level but then brought to the traditional low-block position using the exact same hip movement to generate power. The straight punch follow-up would then be turned into a face-punch delivered a tad low...where the opponent's face is now...
The 'barai' can be a release for a collar-grab as well; again, the movements are the same. Cross center line, minor circular motion, then twist the body and apply the 'low block', which in this case would not block a kick, but take a properly-twisted hand/wrist to knee level, where again a follow-up punch could be applied.
In summary, it is not the basic notion of a 'low block, punch' combination that makes gedan barai so useful. It is the basic movements that are developed when practicing this technique, which open up a world of techniques beyond the simple ones I've described here. The body learns to smoothly intercept by crossing over (seen again and again in many Okinawan kata in various sorts), to apply power from the hips to turn the opponent, open him to counter attack, and off-balance him. It is the movement that is most important, because it builds the power, speed, balance, and confidence necessary to apply this technique in a wide variety of ways.
Gedan Barai is much more than a simple low-block, punch. It is a mistake to dismiss it as a useless defense against an attack one would never see 'in the street'.
. and blocks help. 
