I keep saying, "Don't grip with your knees." And what in the world does THAT mean?
OK, after much Youtube surfing, here are two comparisons for you. (I have to admit, it was surprisingly hard to find the sort of video I wanted, with a young beginner posting the trot, soles of the rider's boots flashing in the sun, and lower legs swinging everywhere. The one I found is almost on target.)
When you sit on a horse, your legs are open, and your first impulse is to pinch with the knees to get your legs back together. No. Don't do that. It creates saddle sores for one thing, and for another it pops you OUT of the saddle and makes your lower leg fling itself all over the place.
The girl in the first video has her stirrups too short, but watch it for a sec. Watch her knees, calves, and stirrups. Don't watch the whole thing, just the first minute or so until she starts cantering over jumps.
OK. Check out the posting. She's holding on with her knees and standing UP in the stirrups when she posts the trot. I know this because her ankles are jiggling all over the place and her stirrup irons swing out (when she's standing) and back to the horse's side (when she's released pressure on them). To make matters worse, she's wearing spurs. Agh. So every stride her knees grip, her feet swing out and her horse gets spurred. Noisy leg and unstable seat.
The shorter your stirrups are, obviously the less contact you have with the horse until you're in jockey mode where your knees are in your chest and your butt never hits the saddle at all.
If you lengthen your leg and open your hips, you can learn how to grip the horse with your entire leg. Think of sitting on a barrel. If you wrap your legs around the barrel and grip the UNDERSIDE then you're sucked down onto the barrel. Grip with your knees and it's like gripping a bar of soap in the shower. Whoops! Off you go.
Then when you get to jumping, you can lift slowly and securely out of the saddle using the strength of your legs, not by standing up in the stirrups. If you go over jumps with your knees pinched to the horse, your lower leg swings back and away from the horse's side. And you've got no stability as the horse leaves the ground. Quite dangerous.
In this video, watch the horse acting up at the beginning. Ignore his actions for a minute and watch the rider. Especially her lower leg. I'll comment again after you've seen it.
She gets left behind his movement only once in all that leaping around. And her lower leg doesn't budge. Sometimes she's in the saddle trying to move him forward through the rearing, sometimes she's hovering just above the saddle trying to keep her center of gravity over his, but her feet and lower leg stay still. It's quite a testament to her ability that she can do this.
And then there's this girl. Watch the whole video to see the same horses after she's worked with them. As the horses act up, watch her upper body stay straight and tall. Watch her lower leg stay pretty quiet. Especially with the bucking dun, you can see how her HIPS are the fulcrum here, not her knees. What an excellent rider she is.
Long and low, lengthen into the horse's movements and you'll be a lot safer up there. And you'll get fewer saddle sores. That's why God made BOOTS for your CALVES not knee guards for your pinchy-thingies!