I thought I remember that you want to progressively squeeze harder with your thighs as you do your rundown to pick up speed, and then remove pressure from your thighs and let your back, seat, and legs "go limp". Of course, keeping little to no rein contact on their mouth because you don't want them to gap their mouth or lose momemtum by raising their head. Although you may need to provide some cue when training, of course.
Is that totally wrong?
And of course, no sliding stops unless you have the proper arena ground and the proper shoeing.