...So, am thinking, I tried to post this guy's trot, but with little success. I wonder if I am having the same issue of falling back during posting, and that would cause me to have an even bigger bounce, yes? Bigger than if I were gently lowering. So I'm wondering if figuring what's going on in two point would help. Just thinking that if I'm falling back into the saddle as he is coming up, I might be getting an een bigger bounce than normal...
I'm not sure how modern hunt seat is done. For a forward seat, in theory, your weight should be balanced above the stirrups. The rule I've read and that makes sense to me is that if you are leaning forward the right amount, you can go from sitting to standing balanced in the stirrups by simply uncoiling your body.
If you cannot, then you are behind the horse's balance - which is good for some things, but it isn't a forward balance and it turns posting into a forward / back motion instead of simply uncoiling. In a dressage seat, your heels would already be under your hip and shoulders, so you wouldn't need to lean forward. But a forward seat, by definition, means your balance is forward to match the horse's center of gravity. That is why I think being able to ride balanced while standing in the stirrups is an important part of being able to post correctly in a forward seat.
Being constantly forward tends to be tiring on you, so most will ride flat ground with their weight behind the vertical line of the stirrups. In a lot of the pictures I've seen of folks riding a modern hunt seat, they are either behind, or they have brought their heels back.
The dressage position is properly balanced in synch with the horse for a collected gait, so a lot also depends on how collected your horse is. In an extended trot with lots of forward motion and little vertical motion by the horse, you need to lean forward more. The more the horse collects (moves his balance to the rear), then the more your heels and shoulders need to move back. Ideally, all riders will constantly adjust their balance to either follow the horse's shifts, or to ask the horse to shift his balance - a fluid balance with a fluid seat.
If you grip with your knees, everything I wrote is no longer true. I think people tend to grip with their knees because they are NOT in balance with their horse and able to switch to 2-point by simply uncoiling the body.
Since I use a western approach to the reins, I need to be able to ride in 2 point with one hand at my side and one hand on slack reins. That also helps prevent me from using the reins for balance, which is one of the few universal sins of riding. BTW - I'm not an instructor or anything. I do read a lot, and what I've written matches up with my own experience in riding.