I recently started riding lessons after about 8 years of just summer camps, and it's been going surprisingly well. :)
But in my last lesson, there was a problem. The horse was being plain lazy and would not hold up a good canter; this ended up being what we worked on for the whole lesson.
The instructor told me to use my leg, not my seat, to control the canter, since using the seat tells the horse to slow down and collect. She said to point my toes out, dig (for lack of a better word) my heels into the horse, and to use my lower leg while keeping the upper relaxed. It just didn't make sense to me. When I tried as hard as I could to squeeze with my lower leg, I just pinned myself down to the saddle- bringing me back to the 'slow down and collect' thing.
So after my lesson, I was thinking about it and all the mechanics, and thought maybe my problem is I need to try a half-seat canter. In my first lesson when I attempted the canter, I was much more comfortable sitting it (now that I think about it, I've probably always sat the canter) so the instructor let me. I tried the half-seat position a bit but it felt like I had a chair seat or was just unbalanced somehow. Maybe I confuse two-point with half-seat?
If someone could explain a half-seat canter, that would be great. Should you be touching the saddle at all? How is your upper body supposed to be?
To achieve the 1/2 seat, light seat position - you have to be balanced over your feet, first and foremost. Find your balance there first. Then when you do, focus on now gripping or pinching anywhere in your legs, and to allow your bodies weight to flow down from your head, into your heels, so that they can be your anchors.
When you find your balance, and your anchors - then you can lift yourself slightly out of your tack, where your seat is just hovering over your saddle. Tuck your seat bone under you, straitening your lower back, and activate your core. Without your core, you cannot control the tempo and rhythm of your canter.
Tall upper body, opened chest - as though you are lifting your heart. Your legs, are asking your horse to come up into your seat every upstride of the canter your horse makes - "come to me" - your legs aren't mainting the canter, your core is.
Your core is was makes or breaks the speed, tempo and rhythm.
I hope that helps.