I'm curious because I've heard the term 'feel the floor in your hands' and some similiar stuff around here lately and that is essentially 'weight underside' IMO. I find weight underside to be one of the more difficult things to explain to people and teach to them so I'm always looking for other ways to do it, as I'm sure are others. Anything anyone has to add would be nice.
The general idea is to let Heaven and Earth pull from each side and Man is in the middle. Heaven and Earth should do the work.
If I hold up a teacup or a load on the top of my head, I should be letting my body structure transmit that load relaxedly to the ground so that the upward push of the ground is doing the work... all I need is a cohesive structure that holds my path to the ground in place. Since the solid surface of the ground is doing the work, I am in harmony with the universe. Someone pushing in from the side, etc., I also just form a path to the ground (it becomes automatic with practice, as you know).
If I am trying to exert a force downward, I want gravity to do the work. I.e., the weight at the center of my body. So I have to very relaxedly (and very lightly, at first) train the weight of my body to be where I want it. If someone is lifting up under by armpits, I have to allow them to hold the weight of my body which hovers just above the crotch area. If they lift up on one of my horizontal arms, I have to let them be lifting up on that same center of force around the lower dantien and crotch area. After a while, any upward push on my body.... which can only be done on an underside surface, of course
... will be immediately connected with the weight at the crotch/lower-dantien.
As I get more skilled, these instantaneous referrals of forces to either the ground or to the weight (on the underside surfaces) become automatic. At that time, I am always "extending ki" in all directions.