"But not falling backwards onto her steps, or he falling forward."

If you look with more care, you will see that before their moving feet are placed (in fact, you can see that they they are a few mm off the floor and thus cannot possibly be bearing weight), their bodies are entirely past their standing feet. Drop a line down Jonathan's back and it is in front of the only foot supporting him. Drop a line down Melissa's front and it is behind the only foot supporting her. Thus technically they are indeed off-balance and "falling" though you might prefer not to use the later word, since they are falling precisly on the path they intended to take.

"And what is moving first. Dare I say the legs."

Yes, that's the most obvious prolbem with their dancing, and is why you can often see thigh-to-crotch collisions in it.

"Compression of the knee forward. Do you know how to do frame by frame. If you do you will see the knee out the back of the man. Now try to tell me it isn't forward."

You had better look again! look at it against the backdrop as you click frame by frame and you will see that in fact it has not moved forwards in space!