Basically when you go into your X step, you reach back. This puts your shoulders facing roughly 180* from the target. It all starts coming out of the X step; the final step starts your rotation out; (Plant foot). As you start to rotate, through body positions, as you're not actually moving your arm yet, the disc is somewhere around your left shoulder. Now, once everything rotates around and your chest is roughly 90* from the target you have a slight pause in your chest, and at this point you "close" your wrist pulling the disc toward your forearm. This puts the disc directly at the center of your chest, (DON'T PULL YET!)
Now during this pause after you close your wrist, you finish rotating your right shoulder. IF you closed your wrist, the disc has no choice but to be "tucked" into the right pec; your right elbow should be pointing at the target. Achieved my rotating your shoulders to line up with your chest. ALL this was achieved by nothing but body positioning, and you should have exerted no force with your arm at all.
Since all this is happening in a split second, it's easy to take body positioning out of the equation and just pull it through with your arm, as that's what feels natural, but that's why you lose power.
This is where the "right pec drill" comes in, as everything from this point after doing the drill should feel 100% natural if you've done the drill long enough. As this tucked feel; with ALL the acceleration from your forearm comes AT THIS POINT OUT. As if you're doing the right pec drill, so that the forearm slings out. The wrist opens to neutral; (Straight with your forearm) due to inertia, then you should physically assist the wrist opening to the "full open" position; (past straight with your forearm) and the disc will then be ejected due to the incredible force you just generated. There is more as to how and why the disc actually leaves your hand, but I won't discuss that right now. But, the split second the disc leaves your hand, you go into the follow through; the arm goes right, and the chest is then pulled to face the target.
So, to recap, everything leading up to getting the disc tucked into your right pec is ALL body positioning and you don't really use any muscles from your arm to "pull" the disc into the pec. THEN, the acceleration gained by your forearm, coupled with the inertia of your body rotating, generates a "whip" like effect, and propels the disc forward.