Pictures might help. If your heels are under your hip, you can extend your leg - I like to think of it as uncoiling my leg, since that just sounds less stiff - and you don't create a forward/aft movement of the stirrups because your heels are always under your hip.
If you have a jump saddle instead of a dressage saddle, then the stirrups will be a bit forward and the style of riding changes to a forward seat. In that case, you are leaning forward from the seat so that your center of gravity remains over the stirrups. You can still uncoil your leg and even get fairly high out of the saddle and it is OK because your center of gravity stays over the stirrups.
If your stirrups are in front of your center of gravity, then you have a 'chair seat'. That is fine if you are about to rope a steer, but not so good for English riding. However, if your saddle is too big for the length of your thigh, then you will tend to have a chair seat. It simply means the deepest part of the saddle, where gravity tries to put your rump, is far enough behind the stirrup leathers that your legs angle forward to find the stirrup.
In a perfect world, you would simply change your saddle to one that fits you. However, most of us live in an imperfect world, and have to take what we can get sometimes.
For illustration, below is a picture of my daughter posting in a western saddle that is a couple of sizes too big for her.
If she sat down, her thighs would be almost parallel to the ground, and her knees bent. Her rump would be well behind the stirrups. When she posts like that, the horse doesn't have enough thrust with his rear to shove her all the way forward. So she tries to force it.
As she forces her hips forward, it pulls her feet back. You can see the stirrup leather are not hanging straight down, but have been pulled with her feet. Now she is actually off balance, and when she comes down, she'll come down hard - annoying the horse, and defeating the purpose of posting. Happily, she has grown a few inches since this picture, and she now looks a lot better.
That is a fairly extreme case, and I would hope your English saddle fits you better. However, she could have posted smoother even in that saddle. The point of posting is not to stand in the stirrups, but to take pressure off of the horse's back for a little bit. If she had been content to just uncoil her legs a little and accept that she would not ever get completely above the stirrups, she could have posted OK.
In that case: If she would keep her knees far enough apart that she doesn't create a pivot point with her knees, and uncoil her legs just enough to get her crotch off the saddle by an inch or so, her posting would have been OK even in a poor fitting saddle. It becomes more of a roll on the thighs than a standing in the stirrups.
If you don't have a chair seat, then you are probably gripping with your knees.