If you are chasing the wrong thing, because a strike deceived you then you'll never have enough time. That's why people get kicked in the face when they reach low to block a kick that they think will hit their leg. Once you make one decision, it takes quite a bit of time for you to be able to reverse that decision and make the correct. By the time you realize you made the wrong move, you'll only have enough time to tense up your face and prepare for impact.
When a long fist circular punch is done correct, it will travel outside of your field of vision and your brain will stop processing it all together, literally forgetting that it's a punch. I've been able to easily hit people with slower punches.
When done correctly the punch hides behind the body or close enough to the body so that your eyes don't pick it up. That's what is happening here.
In addition to that. Trying to grab circular punches is not the best way to approach them because of the path that they travel. An attempt to grab the punch will most likely fall short. Linear punches go straight so all you have to do is intercept a linear path. Circular punches can change the path that the travel.
If you were to compare a linear punch it would be the x-axis. Traveling in a straight path. Let's say 0 represents The person punching. It is the origin of the punch. Lets say at (1,0) represent inches away from the body that the arm has traveled. So with a linear punch, by the time it moves an inch away from the body, that's the path that it will continue to take to a person's face at (7,0) Now lets take a look a the wave. The Sine wave represents a long fist technique. From 0 inches to 1 inch we can see that their are 2 different circulars punches takes a different path, but they share the same point of travel at (1,.08) So at 1 inch away from the body is where that path of the punch can change directions. Lets say the steep wave is an upper cut, the bigger wave is targeting the head. So I can start with a punch that looks like an upper cut and then change it to a punch that attacks the head.
Because circular punches can loop back without stopping, I'll be able to attack left or right using the same arm. In this graph, the left side is above the A axis. The right is is below the x-axis. If my circular punch comes from the left side and travels right, then when it gets to the right side. I can make it to the opposite of what it did on the left side. If my Palm is the impact surface going left to right, then the back of my hand with be the impact service going right to left.
The problem with circular punches is that you don't know when that of the punch will change, unless the person only knows how to throw one type of Circular punch. If all you know are big circular hay makers then it's easy to find the path because only one path exists.