The weight has a recognizable "center". It won't move. If you are strong enough, and can "sense" where the gravitational pull is on the weight, you can move it. But with a 100 pound guy, he can hide his "center" while maintaining a downweight anywhere on his body. So the guy will not be able to sense where the pull is on the 100 pounder, while the 100 pounder is mentally directing gravitional force affecting his body and consequently aligning his body in such a way that it desires to stay clamped to the ground.
I don't know if that makes sense. Jeez this is hard to explain.
I remember someone years ago telling me to visualize being a loose sack of sand but with the ability to shift the sand at will. So to be in control of it in a relaxed fashion. So you decide to shift over here, over there, etc. as the person grabbing is trying their hardest to find a "thing" to lift. So no matter where they start to lift you're "pouring" over here, over there, etc. leaving them with nothing solid. Not giving them that center you talk about.
Or to put it another way, I think of using aiki to feel their center while not allowing them to feel mine. Or to keep mine controlled and invisible to them. So I can affect them but they can't affect me. Kuzushi on contact via aiki.
Yeah, while really hard to explain it is so much harder to do (for me at least). Still learnin'...