If a horse warms up with weighted boots, it is harder for him to lift his legs with a 1 pound weight on each leg. So, when he goes in the ring, without the boots,it is easy for him to snatch his legs up higher that he might do otherwise.
The boots are legal for warm up, just not in the ring. So you might see a horse’s boots being changed out at the back gate – this is why.
From what I have seen, it is a training gimmick that is losing its proponents, although some people swear by them. I am neutral on the subject — I do not know if they work as advertised, but they do not hurt the horse, so if people think they help, then let them use the boots.
They are similar in theory to all the gaited horses who warm up with stretchy bungees attached to their hooves. It is harder for the horse to lift his legs with the bungees (it is a VERY stretchy bungy which just provides a little resistance at each step). Then at the back gate, the bungees are removed and the horse, now without resistance snatches his legs up very high.
Silly? Maybe. Harmful? No. Painful? No