A yoke is a very simple type of breastplate that consists only of a neckstrap and a strap between the horse's front legs. By definition, a yoke has no connection to the saddle or bridle and does not affect the horse.
Yokes are most often seen in racing but occasionally in eventing. They prevent the girth from slipping back, but their primary purpose is to give the rider an extra "handle" to grab in an emergency. In some areas they are called "chicken straps."
They should not be confused with the yoke used on oxen.