Originally Posted by freia
Martingale: German martingales can be used under training to "adjust" a headset. Good hands and a skilled seat will do the same. A running martingale is very useful when jumping a horse that pops its head up so the rider doesn't get hit in the face. I'm sure there are other applications as well. I'm not familiar with the use of a standing martingale. Common for all of them is that they're used to correct a headset issue. Sometimes they're used to hide a problem that a trainer or rider can't figure out how to fix correctly. Sometimes they're used (correctly) as an aide to help a horse stay in a better headset - combined with correct use of hands and seat.
That is NOT a correct use of a martingale, the correct use of a martingale is to stop the horses head getting above the angle of control. It should never be used to put a horse in a "headset".