Sit when the horses outside leg is on the ground
You rise as your horse's outside leg reaches forward
Rise and fall to the leg on the wall
If you can't quite figure it out when riding. Try closing your eyes trotting and feel whitch knee feels slightly forward and look down to see what diagonal your on to learn the feel of each one.
When the outside leg goes forward you post
When the outside leg goes up, you go up, and it goes down, you go down
When the outside shoulder goes up
The horses outside leg should go backwards when you sit in the saddle
When trotting and on the correct diagonal yours horses outside leg or leg closest to the rail. That leg should be forward when you are out of the saddle. And to change your diagonal you sit two beats .