One of my earliest recollections of riding lessons, at the tender age of 4 or 5, was having “UP, DOWN, UP, DOWN” shouted at me as I tried to master rising trot.
My shoulders aren’t in quite the right place, it’s a work in progress, but at least I’m not rising too high!
Just rising forward a tiny bit is a much better bet, it looks far tidier and lets you control the horse’s rhythm better. If you then want the horse to do bigger strides in lengthened or medium, you can try doing bigger rises to encourage the stride to open.
Something my trainer says which I always find helpful is “Let the horse rise you”… i.e. don’t make an effort to rise, just let the horse’s movement push you forwards/upwards a little.
Let the horse throw you up in balance, but dont forget your horse has to balance under you in the sit part. Some horses tighten their backs to force you to trot at their rhythm not yours. The easiest way to teach someone to rising trot is to uses a neck strap. It will keep the rider travelling forward and at the same time teaches them to keep their hand still with soft elbows.