Hypothesis 1- The muscles of the dantien (muscles in the hip/abdomen area of the body) are the only muscles moving the body.
Hypothesis 2- The muscles of the dantien (muscles in the hip/abdomen area of the body) are starting an action that makes a sequential contraction in muscles from the dantien area out towards the localized muscles, "chain linking" muscular power together.
Hypothesis 3- The muscles are not being used much at all, something else in generating force.
This probably isn't even worth two meagre cents, but the idea of dantien comes from a tradition that did not think of the body as a system of joints an muscles. So although I find myself agreeing the most with hypothesis 2, the mentioning of 'muscles' makes me nervous, because it makes no sense in the model of the human body the idea of 'dantien' is coming from and secondly because in my own practice I focus on relaxing all muscles as much as possible.