One a day, plus iron

A trot is a two-beat gait where the horse's left front leg and right hind leg work together and the right front / left hind work together. Trotting is a two-beat gait. It's kind of bouncy. In order to mitigate the bouncy factor, people who "English" post at the trot. This means that they rise out of the saddle on one of the diagonals and sit down for the other. This gives kind of an up-down-up-down motion to their riding.

English saddles (and some endurance/competitive trail saddles) have stirrups that are hung so that the rider's leg is in a good position for posting. It's a point-of-balance thing, I think. The pivot for posting is actually the rider's knee.

Western riders (cowboy hats, rodeos, cows are involved, big horn sticking out of the front of the saddle, etc.) do not post the trot and their saddles are not designed to accommodate it. The stirrups are hung in the wrong place so that the rider who tries to post in most western saddles finds his or her torso flung forward and his or her feet sticking out behind. It's just wrong.

More information on posting is here (http://lorienstable.com/articles/riding/400-trotting/)