Drilling (Tsuan) is one of the 5 Fists of Xing Yi however it is also a quality that should almost always be present.
This is a twisting spiraling force through the entire body in each movement.
Down through the legs into the ground. You’ll feel pressure on the outside edges of your feet as they try to turn outwards (but don’t let them)
Up through the body and out the arms. You should feel this spiraling help connect and unify the entire body.
It’s important to know that although you should feel this twisting through the entire body it can happen mostly internally. So you wont always see it in the movement.