Just learned one in my traditional aikido dojo.
When you are on the ground and uke is standing in front of you, hold his foot down flat on the mat with one hand. With the other hand grab his leg from the front just above the ankle. Press inward in the dent on either side of the base of the shin bone. Do it right and he will fall over forward.
Was there any theory taught alongside this, I mean in terms of mechanics?
To me, the example you made does not sound overly practical (it might not have been intended that way). I would be interested to know what you thought about the Youtube clip?