In one sentence, describe what the following article is about:

The main reason to post while riding is to avoid being jostled around in the saddle during a particularly bumpy gait - the trot. This makes things more comfortable for the rider and is easier on the horse's back.  Once you become more experienced with the rising trot, it is also possible to change the horse's rhythm simply by posting more quickly or slowly. The horse will alter his rhythm to match with your own, thus sometimes eliminating the need for hand and leg signals. Urge your horse into a steady forward trot. Now, pay attention to the horse's strides - notice how the trot has two beats? Good. This will help you to understand how posting works.  During a trot, the horse's left hind leg and right front leg (which are diagonal to one another) move simultaneously, and vice versa. This is what riders are talking about when they refer to "diagonals" - the simultaneous movement of diagonal front and back legs. These diagonals will greatly influence your rising trot. When performed correctly, the rider will rise from his/her seat when the inside hind leg and outside front leg are moving forward and sit when the outside hind leg and inside front leg are moving forward. The reason for this is because the horse's inside hind leg is the one which propels him forward. Rising out of your seat while this leg is in motion will encourage the horse to stretch the leg further under his body, and therefore producing a more effective stride. At first, it can be difficult to tell which diagonal your horse is on. A good trick is to watch your horse's outside shoulder. You should rise out of your seat as his shoulder moves forward and sit back down as it moves back. If the movement is difficult to pinpoint, stick a bandage or piece of colored tape on the horse's shoulder. This makes the movement easier to see. Once you start posting your legs will likely be sore, but you'll gain more thigh muscles over time.

Summary:
Understand why posting is done. Understand diagonals. Realize that posting should be done using your thigh muscles, not your legs.