Same principle as in other athletic endeavors like powerlifting where you do a slow controlled decent of the weight (benchpress and squat) to build up that potential energy at he bottom of the movement for an explosive release.
You are correct in that the triceps muscle is used to move the cue backwards but it
also plays a vital roll in stabilizing your upper arm when the biceps is active in the
forward motion of a pool stroke. The stability is both lateral(side to side) and also
vertical in coordination with the rear deltoids in your shoulder. The reason that a
slow back stroke can work so well for those who use it is, that by
purposefully engaging the triceps it adds even more to the stability of the
smaller biceps muscle in motion and additionally assists with the potential
energy being stored for release in your stroke. So a controlled slow pull back
is loading the triceps and biceps muscle with potential energy to later be
released as kinetic energy propelling the cue through the ball. The stability created
by the triceps helps to maintain control over the direction in which the biceps releases
that energy. The biceps and triceps work in coordination with each other and even in
strokes like Allen Hopkins that punch the ball with very little backward motion both muscles are
still playing their respective roles even though it appears to be mostly the action of his biceps.
Hope that makes sense and helps. Good shooting to you,
Kevin