I think for young riders and letting go of the saddle it's all about confidence. Do they feel safe and secure in the saddle? To feel this way their balance needs to be centered with their heels down.
Good answer little creek stud
with our young ones we get them to stand in the stirrups at halt then walk as long as they can (usually no more than 10 seconds) than once they have a general feel for trot we get them to play 'simon says' and tell them to put one hand on their head at trot and the other on the pommel then progress to hands on head,arms out,hands on knees ect.
Out them on a lung line and tell them to look at a front leg and go up as that leg goes up and go down as the leg goes down once they have learnt how to do it tell them to take their eyes away and get them to understand the feel of it