Impulsion

light, elastic thrust of the pony’s movement. It creates the power of the collected gait necessary to perform highly disciplined movements of the higher levels of disciplined training. Movements such as the passage, piaffe, pirouette, tempi changes, and the collected gaits, as well as the extended gaits, rely on this principle. Impulsion is also associated with the suspension of the trot and canter. The emphasis of the movement is forward.

*taken from - The Human Pony: A Handbook for Owners, Trainers and Admirers found at http://www.greenerypress.com/