To post on a
trotting horse is to move rhythmically up and down in the saddle in synch with the movements of the horse. To describe it is to make it sound a lot harder than it is. (Try to tell someone how to
skip down the street! Describe riding a
bicycle! It sounds difficult, but it isn't!)
The trot is a two-beat gait which involves the horse's hooves hitting in diagonals: right front with left rear, left front with right rear, over and over. In other words, the horse goes bounce/bounce/bounce/bounce as the two diagonals hit the ground alternately. To post, one lifts oneself up in the saddle just a little on one bounce, then settles back on the next, then up again, then down again. It sounds harder than it is: if you match the rhythm of the bouncing horse, in fact the horse is doing all the work, and you are just compensating a little, smoothing out the motion for your and the horse's convenience and comfort, using the motion of the horse to move gently up and down. It feels like the most natural thing in the world.
Horses can only canter or gallop for short distances, but a horse in good shape can trot for hours. Hence the trot is the gait which was used for the carrying of the mails. It is probable that this riding technique takes its name from the post riders, who carried mail for long distances on horseback. Webster's next to last definition for "post" is "to travel (by horseback of course) in haste," that is, trotting.
Sitting with your butt flat on a trotting horse for hours makes most people feel as though the teeth are going to come out of their mouths, whereas after you catch the hang of it you can post nearly indefinitely in comfort. It's better for the horse too. If you just sit there, you're going to bump up and down and the repeated blows will tend to make his back sore. Who cares? You care. I was taught that the comfort of the horse is your safety. It's true. Make the horse's back sore enough, and he's going to find a way to get you off of there. Since he weighs about 1,000 pounds, guess who wins that contest!
For some reason I'm not sure I understand, cowboys, and the real old-timers, countrymen my age (that is, in their 60's, and older) all absolutely refuse to post. This probably has something to do with the old-timers' idea that English style riding (with which they identify posting) is effeminate. I have yet to meet a real old time Western rider horseman over the age of 40 who had anything but contempt for posting. But watch them closely on the trail and you'll see that they're cheating a little. To spare themselves and their mounts they tend to stand up just a tiny bit in the stirrups (called in English "riding two-point"), steadily, not moving up and down, but not bumping along either, and using their knees and ankles to cushion the repeated blows on the bouncing horse.
If you're really good at it this probably works just as well, but most beginners find that posting is easier.