I have a weird feeling that this photo is deceiving. At first glance, it looks like he is driving forward with his head. But take a second and look at the feet: his weight is obviously on his front foot, which would make a forward movement fairly powerless - not typical of O Sensei at all.
I think he is actually pushing off his front foot, dropping back and bouncing uke into the hole that his body is leaving. That would make the head position seem very natural.
I hope uke was feeling sharp that day.
