What is Sakaki's Heian Hyakudan?
The technique Heian Hyakudan is non existing technique in karate. 

Shio Sakaki claims to hold 100 dan in the arts of karate . So the move Heian Hyakudan could be a word play on this with Hyaku meaning 100 (百).

As for his movements, with the rotation and forward moving speed he should have fallen indeed. Although in the manga (don't remember the exact chapters) it is also shown that Sakaki can alter his central point of gravity by using certain muscles. Which in this case would have helped him with maintaining balance and allowing him to land in such manner.