For shortening and lengthening under saddle it is a combination of aids which can be described best with you on the horse, by a coach. For me, it is mostly seat and only a supportive leg and hand.
On the lunge, I like to teach that the horse only changes gaits when I say "walk", "trot", or "canter" and that clucking is for more forward or lengthened stride and a tongue roll (the weird "brrrrrt" noise) is for slowing down or shortening the stride. It takes a lot of corrections and praise but the voice aids become useful undersaddle as well.
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