I think I understand this one now. I had to do it a punch of times to keep it from being kung fu. Those pants threw me off as well. I thought he was stepping forward but didn't realize it until after I got enough of the Kung Fu out of the movement.
So this is what I'm seeing in this video based on what I know. I'll start with the feet.
1. He's stepping at a"45 ish" angle, which means he's stepping off center. If he's stepping off center like that then it's because he's dealing with an incoming jab. You guys have actually seen me do the same foot work. The biggest different is that he walks into the angle and I shuffle into the angle. The challenge of walking into an angle is that you have to be good with seeing the distance between you and your opponent, If you aren't careful you'll walk into a punch.
As I did this, I'm imagining a jab coming in and I'm stepping off the center.line. This only works if your hands are in a fighting stance. My forward movement triggers the jab (hopefully). My left hand must shoot between the jab so and my striking hand so that I can redirect the jab. If I can redirect the jab, then I will have an open shot to the back of my opponents head. Probably right behind the ear. I'm basing this on the angle of the strike and how his left arm treads the space between the attacking jab and the open hand strike.
Doing this movement from a fighting stance with your guard up gets rid of the telegraph strike. I cold be wrong about this as I don't do Aikido, but I've hit people with strikes from similar movements. The fact that his foot work looks very similar to my footwork makes me think that's what's going on.
I went back over the foot work and walking forward makes it easier to thread that left hand. In the clip. That strike works better if your attacker has his left hand forward, power hand back. Which is how most right handed people stand. The step is probably smaller in application than what we see here.
What do you think GpSeymour? Give it a try and see if it feels that way when the technique is used like that.