I looked for a video with some explanation, but couldn't find it. This video has a different throw, but the principle of using the arm to get the shoulder down and across is the same. Interestingly (to me), this one's titled "2 on 1", which is the name of one of the techniques that teaches this principle in NGA.From what I can see, it has a short lead in the other direction, then the pull on the arm leads the shoulder down and across. That down-and-across lead shows up in a few techniques we have from Daito-ryu. If the foot on that side (the same side as the arm) doesn't have a chance to compensate, it results in a surprisingly fast downward fall from the shoulder. The purpose of the short lead to uke's left (the arm in question) is to both generate some momentum and to set some weight onto the left foot so it can't compensate. You can see those principles in aiki arts, but seldom will you see them get to this kind of application - it just doesn't look as pretty, and all the set-up time doesn't fit the aesthetic of most aiki folks. But this is what aiki should be like, IMO.