Control Of Rising Trot Tempo
The ability to produce impulsion and direct the horses energy is down to how well you can keep taking the horse and maintain a tempo as you ask for energy. Riding the horse forward without tempo control leads to running, hollowing of the horses back and about zero chance of balance or getting the horse to work on the bit. This video goes some way to explaining tempo control in rising ( posting ) trot.
Let me know if the video helped you in the comments below the video.