Understanding stability and grounding it can then be taken advantage of, both to generate more power as nage and to exploit the body positions of uke to make them weaker
The Base of Support
Applying this to people and the accepted hanmi stance of aikido. The Base of Support is a shape joining up the circumference around the feet.
One direction has a bigger base - this is the usual direction of applied power where we might punch from or direction to cut a sword into.
The other is the position that we seek to get to because the other person cannot apply or receive power as effectively. The omote and ura body positions begin to exploit this - although the ura position needs a few extra tricks like heading to shikaku (the dead space behind uke)
other manipulations include changing the dimensions of the base of support. e.g. an uke up on toes has an extremely narrow base of support.
How to get them there is the trick addressed in part in the toppling section, though clearly joint locks like sankyo and waza like aikiage are also tools for this
It follows then that as long as someone weight is firmly between their legs they will need to unweight one to move a leg. Moving a leg under certain circumstances is the beginning of a fall. By freezing the base, through compressing uke down onto feet or stretching out uke upwards but keeping weight centred they will be unable to move. The weight can then be shifted anywhere along the red line with the same result