Posting is to rise out of the saddle seat for every other stride of the horse's forelegs, thus smoothing out the jolts you will encounter if you just sit. This makes riding the trot much more comfortable for you and your horse. It doesn't matter if you ride English or western.

Why do Western Riders Post?

Posting the trot is a way for the rider to stay in rhythm with the horse or in some cases to establish a steady rhythm for the horse. Imagine how difficult it would be for the horse to trot steadily if the rider was bouncing all around at random.

Why do you post with the outside leg?

It refers to the correct diagonal as being when the rider is sitting when the horse's outside front foot and the inside hind foot are on the ground. This is commonly referred to as the “outside diagonal.” ... Therefore, if you always post on the same diagonal, your horse will develop strength unevenly.