Some of it depends on your philosophy of riding. Some people say there should be little or no pressure on the stirrups. To post like that, all the weight needs to be carried in the thigh. I started my riding learning a forward seat, where weight (pressure - you cannot have one without the other) is encouraged. In that case, using the stirrups makes sense - provided your center of gravity is over the stirrups. If your center of gravity is over your stirrups, then your lower leg stays stable and posting is a small up/down motion - because you do not need to come forward in order to be balanced over the stirrups.
From that perspective, as Paul Cronin points out, posting or two point done stirrupless teaches the wrong habits. It requires all the weight to be in the thigh and encourages gripping with the knee, both of which prevent weight flowing into the heels. It may be fun and it may be effective exercise, but I don't want to ride that way as a goal. It is good, it seems to me, for stretching the leg and getting the thigh more vertical.
From a traditional seat perspective, if your heel, hip, shoulder and ear are all in a vertical line, then again - why do you need to come forward to be in balance? You are already in a vertical line balanced above the stirrups. This is Steffen Peters on Legolas in extended trot (“Firsts” at Hagen’s Horses & Dreams in PhotosÂ*|Â*
) If he were to post, why would he need to thrust forward with his hips? He is already balanced...
From a western perspective, my knee when riding is next to the latigo. This is what is between my knee and the horse:
Squeezing with my thighs just raises me in the saddle, leaving me perched on the horse. Pressure with my knees would have nothing to go against. I must
let the weight slide down uninterrupted past my knees and into my heels.
I'm a backyard rider. I teach no one and compete in nothing. When the picture below was taken, I was using what I call two-point (which is the same as being at the top of the posting motion) to work with Bandit on his trot. As you can see, I'm still wrapped around my horse. I am NOT coming 6" out of my saddle and thrusting forward - just unfolded a little, bringing my crotch out of the saddle and my rump off the saddle, but very little gap. After all, the horse doesn't know if I'm 1/2" high or 9" high - only weight or no weight. My personal goal is weight off the saddle, but jeans still touching.
In that position, if my horse throws on the brakes without asking me - which Bandit does at times - I stay steady. I keep this position while turning around pylons - just need to shift my weight back a little to match the change in balance of my horse.
If you think of the knees and hips being hinges, then all I need to do is open the hinges a little and I'm at the top of the post. If I stay there, I'm in two point. If I close the hinge slightly, I'm on my horse's back. If I alternate, I'm posting.
Sit on the edge of your chair with just your hips, and bring your heels under your center of gravity. You can now "post" at your computer desk. If you are like me, you'll get tired fast because there is no motion from the horse helping you.
Now, sit back into the chair. Try to post. You cannot. You cannot get out of the chair without first thrusting your weight forward. If you sit back in a chair, the first thing you do to get out of it is move your shoulders forward so your center of gravity gets closer to your heels. You then intercept the forward momentum and rise. We move forward and rise from our knees to get out of a chair
. I think the same applies to a saddle...
But I'm a backyard rider. My views on this subject seem to be very eccentric. FWIW, though, here is what Gen Harry Chamberlin wrote about testing your riding position. Notice he considers standing in the stirrups to be the same thing as being at the top of the rise in a post:
BTW - these are screen captures from the video tinyliny mentioned:
His stirrup strap is well forward, his toes are pointing down, and his center of gravity is well behind his base of support. Not to my tastes, but I am eccentric in my views...