Thrust. Releasing of the energy stored by engagement. In dressage, impulsion is associated with a phase of suspension such as exists in trot and canter, but does not exist in walk. Therefore, impulsion is not applicable to the walk. [Note: It may be enlightening to compare the original French with the later English translation of the FEI scoresheets under "Impulsion." The English translation translation of the French reads, "the desire to move forward," whereas what the French actually says is, "the desire to carry himself forward" ("Le desire de se porter en avant").
Impulsion is the pushing power (thrust) of a horse, which comes from his desire to move powerfully forward with energy. However, speed does not create impulsion, and a rushing horse is more likely to be "flat" than impulsive. A horse can only have impulsion if his hind legs are engaged and his elastic back allows the power to come through.