Aikido video from my school that was filmed in slow motion.

[drop bear]

Yep, almost everyone does it. And again how many knife attacks actually happen in a thrust step through into close range like that? Very few. But almost all demos show it that way and then in the real world it doesn't happen. Yes, some is for attribute development but I feel there are more realistic movement and ranging drills that are more effective.
They are all cool to watch though.

in modern western unarmed culture. Possibly different in a knife culture.

medieval dagger.

 

[drop bear]

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weapon fight. Somewhere or other.

 

[K-man]

Does the concept revolve around basic motion? For example, while an attacker probably won't always step through with a punch, the basic motion of someone moving forward and attacking is generally the same. Likewise, someone won't always come at you with a downward chop with their hands, but you could apply that same principle to someone attacking you with a club or baseball bat.

???

Most definitely and I thought after I posted that I should have added a little more. In Aikido, and the principle applies across the board, maintaining your body structure while destroying your partner's structure is really important. As your partner attacks, you maintain your structure and often your position on the line while redirecting your partner off the line. That redirecting compromises his structure or balance enabling the technique.

The downward strike could translate to another attack but in Aikido I believe it is more to do with timing. An empty hand shomen attack can redirect very easily so as nage you can't move too soon. A weapon attack with a heavy object is not quite the same as that involves more momentum. Even so, the defense against a the downward shomen attack with a weapon can be the same particularly with Kaiten nage where the downward momentum is utilised.

Not a particularly inspiring video but you can get the general idea ...

 

[Spinedoc]

In addition to what K-Man wrote above, one of the things my sensei and I have talked about often is that Aikido relies on a committed attack. Remember that Aikido is defensive only. If someone is going to throw a punch but not commit to it, an aikidoka will likely simply step back and stay out of the way. As an example, if someone is trying to fight me like a boxer, and is trying to jab at me, while not committing, well, there won't be a fight, because I'll simply back up out of his way and give him room. It's only when he tries to close that distance, and is willing to commit his attack that Aikido begins to work. One of the reasons we practice with someone stepping in with their punch, is that that represents the commitment to the attack.