My understanding of how one fermion changes the energy of another is something like: fermion x approaches fermion y and x emits bosons which are absorbed by y. But why does one billiard ball transfer a lot of its energy to another billiard ball on contact? Say billiard ball x approaches billiard ball y, x hits y, x stops and y moves. I realize the Pauli Exclusion Principle forbids x from occupying the same place as y. But why does y absorb more bosons from x than vice versa? Why would the fact that x is moving y to absorb more bosons from x than x absorbs from y?