A lot depends on what style of riding you are doing. For a forward seat, it is OK to lean forward. In fact, you OUGHT to lean forward. With a forward seat, you also don't want to absorb the motion in your lower back, since you are leaning forward already.
A dressage seat attempts to absorb the motion in the small of the back:
A forward seat does not:
From the saddle, I'm assuming you want a forward seat, and for that you should lean forward. [Note: if you are training for dressage, ignore this post entirely!
] I also think you should practice cantering in a half-seat, because it is 1) easy on you, 2) easy on the horse, 3) stable, and 4) allows you to feel the horse's motion and slowly sink in and adapt to it.
From an old thread:
Riding the canter in half seat
Originally Posted by maura
Riding the canter correctly and well in a full seat is difficult, and many more riders do it badly than do it well. As Allison stated above, it requires a degree of abdominal fitness, as well as correct position, relaxation and a good understanding of gait mechanics and how the horse's back moves. That's out of reach for a lot of recreational riders. I would much rather see an elementary or intermediate rider cantering in half seat, allowing the horse to move freely, than someone attempting and failing a full following seat and punishing the horse's back in the process.
There is nothing inherently insecure about riding the canter in half-seat or two point as long as the rider is in balance.
Remember - the horse's back barely moves at the withers. At the loin, it moves a LOT while cantering. If your weight is in your thighs at the wither, and your rump barely touching the seat, there is almost no motion to 'absorb'.
I strongly recommend:
It is an older approach to riding a forward seat, but it has the clearest and easiest to follow descriptions I've ever seen. The text in the first picture comes from it.