One of the actions of taiji is to scoop/carve. It is like when you use a spoon to pick up soft/jello-like yogurt. You can put the spoon on the surface and rotate under it to pick up what the spoon can hold. By tossing it in the yogurt will on disturb and destroy the yogurt.
A static version of this is a half-pipe/concave circle. When an object hits it, the object will travel on it and return.
In this video clip, Master Chen Zhonghua shows how to lock the rear kua as the stable support while moving the front kua. This will create a trajectory that will return the opponent’s oncoming force.