A forward seat is not a "jump seat". To think of it as a way to jump is to misunderstand why and how it was used, and still can be used. It is a way to ride, not a way to get over an obstacle. Thus I can honestly say I've ridden most of the last 8 years using a forward seat - because I have.
Weight must go someplace. The options on a horse are limited to butt, thighs, knees and stirrups. Although one can allow the weight to flow into the heels, it is actually supported, of course, by the stirrups.
If your rump is out of the saddle, the weight must go into thighs or stirrups. To prevent it from flowing into the stirrups, one MUST squeeze with the thighs and knees. Unless one squeezes, it will flow into the heel and be supported by the stirrup.
Letting it flow into the stirrup, uninterrupted by the knee's squeezing, is not, in any way, bad riding. Nor does it destabilize the lower leg. Not if you are balanced on the horse. One might as well say a person standing on the ground has an unstable lower leg!
The picture below comes from a book by Jane Savoie:
E & F are balanced positions. Notice what Jane Savoie says (and I echo):
In order to sit in balance, your feet need to be under your center of gravity. Both have the rider's center of gravity directly above the stirrups. In either one, the rider can
unfold a little, not gripping with the knee, and end up standing in the stirrups - balanced! There will then be a lot of weight in the stirrups, but the rider will remain balanced and the lower leg will stay in place. When posting in those positions, one can unfold the body, and raise it and lower it without pivoting forward around the knee.
That is one of the traits of a balanced seat - one CAN raise oneself's to the top of the posting position without needing the horse to push you (creating more work for the horse) and without leaning forward, or gripping with the knee and then pivoting around it.
Picture g has the rider "behind the horse", which is fine as a defensive posture. Many western saddles are designed to put you in that position because it IS a good defensive posture, although it is more work for the horse. Littauer borrowed the following from Chamberlin & the US Cavalry, but that is OK:
"
To see whether he is really in balance with the horse, the rider should try the following experiment; without increase in inclination in his torso and without any lurching up or forward he rises slightly in his stirrups and stays up while the horse walks, without toppling forward or collapsing backwards. The rider's weight is then supported by the stirrups, and this attitude is given stability by the tension in the three springs...This incidentally, is also the rider's position during the upward beat of the posting trot and at the gallop..." - Common Sense Horsemanship
No one is saying you should "brace" in the stirrups. If you brace, you lose two of the the "hinges" that allow you to absorb shock and stay out of the way of the horse - the ankle and knee are gone, and all you have left is the hip. Bracing is bad. Bracing is bad if you are standing at attention on a parade ground, too - you're likely to pass out and fall. Watched it many times.
But putting weight in the stirrups is no more bracing than walking or jogging is bracing.
Now, does anyone NEED to ride this way?
Nope. There are a lot of ways to ride a horse, including gripping with the knee.
But a few things ARE true:
1 - Allowing weight to flow into the stirrups, and even "standing in the stirrups", does NOT destabilize the lower leg. It does not destabilize mine, and I am neither a great athlete nor God's Gift to Horses. So if I can be stable, balanced and secure doing it - it CAN be done. If I could ride out many spooks that way, then it is also secure.
2 - If allowing weight to go into the stirrup destabilizes you, in any way, you are not riding balanced. You are either in front of or behind the horse - which is fine by me if you have a reason, but it is not balanced. You are substituting grip for balance.
3 - If you need to move forward to feel stable at the top of a post, then you started behind the horse - unbalanced, although that is OK if you have a good reason to start there.
4 - Just because a top athlete, riding many hours a day, can successfully do something, it doesn't mean the average rider can imitate it. It is a serious mistake to try to imitate top riders without actually BEING a top rider!
And here is something odd:
Folks are glad to read a paragraph by an anonymous Internet blogger, or listen to a few sentences said by their local instructor, and carry those around for life as if they are written in stone. But QUOTE a great rider like Chamberlin (4 Olympics, twice as team captain), or quote an instructor who taught thousands of students (and who felt he could safely teach someone who had never been on a horse before to jump 2 1/2 feet in 15 lessons)...and well, who cares about them! Imbeciles and morons! The only people who knew or know how to ride are those who lived in the 1600s (Dressage Masters) or those born...well, since we were, be that 1950 or 1990.
If "standing in the stirrups" unbalances you, you were not balanced to begin with. The fault is your unbalanced position, not the weight in the stirrups.