I had the original Sprengers, and then got the Bow Balances, which I like much better. They are wider and seem to give me more stability when jumping. I don't know if they "teach" your leg the proper position, but in my case they have not made anything worse. My trainer borrowed mine and now she wants them too.
Do they help "teach" your leg the proper position?
I think "assist" is a better word then "teach." IMO, it just makes it easier to keep you leg in the right position. You need to put your leg in the correct position to begin with - it is always possible to work against work against your stirrups, just as it is possible to sit incorrectly in a perfectly fitting saddle.
I have the original jointed and another student in my barn bought the bow balance...my trainer told her not to get the bow balance (which she didn't listen) because in a real horse show she would get down graded in eq classes. I tried them once and hated them. I'd say stick with the original...plus if you look hard enough you can find really good deals. I think I found mine for $130 new.
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