A brief, coordinated impulse from seat, leg and rein that rebalances the horse before a new demand — and is ended immediately by yielding.
The half-halt is the quietest and most effective tool you'll learn — and the most misunderstood. It has three parts, and the third matters most: a brief impulse from weight and leg, a restraining rein, then immediate yielding. Leave out the yield and you're simply holding on; the horse learns to lean into your hand instead of rebalancing. You use it before anything new: before every transition, before the corner, before the turn, before a fence. It assumes reasonably stable suppleness and contact — on a tense horse a half-halt turns into a pull very quickly. Have a trainer's eye or a video check your timing, because feel is unreliable here.