English Running Martingales and How They Work
The English running martingale functions by coming up from between the horse’s front legs and fork into two pieces that end in loops. The reins are threaded through the loops. Reins should be complete with rein stops or have rein stops added, to prevent the rings of the running martingale to get tangled up in the rein buckles or bit. If your reins do not have rein stops they can easily be purchased on their own, typically in rubber.
When a horse lifts their head high in response to bit pressure, they successfully disengage the bit and are able to evade the bit or "run through the bit". A running martingale works by applying downward pressure on the horse’s mouth, through the reins and bit, keeping the bit engaged, when the horse tries to lift their head to evade the bit.
The running martingale is popular because it works well when it is needed and has little or no influence on the horse when it is not needed.
As with all horse training tack, if used incorrectly you will not receive the desired result.