Voice cues play an especially valuable role when starting a young horse under saddle because they provide a communication bridge between what the horse already knows — the voice cues established during groundwork and lunging — and what it is still learning, the physical leg and rein aids that the rider will gradually introduce under saddle. A young horse that already understands walk, trot, and whoa from lunging can be given those requests by voice during early riding sessions, which reduces the number of new things the horse must figure out simultaneously.
The practical application in early riding sessions looks like this: the horse is warmed up on the lunge and the voice cues are confirmed working before the rider ever mounts. Once mounted, the rider uses the same voice cues as during lunging — trot on spoken in the same rising tone, whoa spoken in the same slow descending tone — supplemented by very light physical aids. The horse responds to the familiar voice cue and the physical aid reinforces the response. Over sessions, the physical aid is progressively lightened as the horse learns to connect the ridden physical cues to the gait transitions it already knows.
This approach significantly reduces the confusion and resistance that can arise when young horses are started under saddle using only unfamiliar physical aids with no vocal bridge. A horse that can connect the rider's leg aid to the trot it already knows from the voice cue makes the transition to leg-responsive riding faster and with less anxiety than one for whom everything under saddle is entirely novel.
Ken McNabb's colt starting work explicitly uses voice as a bridge — establishing clear voice cues on the ground before the first ride and using them deliberately in early sessions to help the horse understand what is being asked before the physical aids have been fully taught and confirmed.