What is the role of the outside leg in asking for a canter lead departure?

The outside leg is the primary driving aid in a canter lead departure, and understanding why it is the outside leg rather than the inside leg that initiates the canter reveals the biomechanical logic of the aid and makes it much easier to apply correctly. This is an aid that confuses many beginning riders, who instinctively feel that they should push with the inside leg to produce an inside lead departure — but the mechanics of the canter make the outside leg the more important of the two. The canter begins with the outside hind leg — on the right lead, the left hind leg is the first footfall in the stride sequence. This outside hind leg pushes off the ground first, creating the momentum that allows the diagonal pair to land second and the leading foreleg to reach forward last. Because the canter is initiated by the outside hind leg, the aid that asks for the departure must influence that outside hind leg to step forward and push with energy. The outside leg of the rider, positioned behind the girth, is anatomically placed to do exactly this — its position behind the girth corresponds to the location where the horse feels leg pressure that asks the hindquarters to engage, and its direction of application sweeps or bumps the horse's barrel in a way that drives the outside hind leg forward and under. The outside leg behind the girth does not push the horse sideways or ask for lateral movement — it is a clear forward and under signal specifically to the outside hind leg. A horse that is trained to respond to this aid will step the outside hind leg forward into the canter sequence promptly and clearly, initiating the departure from the correct hind leg and producing the correct lead as a natural consequence. A horse that does not yet have a clear understanding of the outside leg aid will be confused by the departure cue and may pick up either lead depending on which is more comfortable, which the rider inadvertently set up through position, or which the horse happened to be stepping with at the moment the aid arrived. The inside leg remains at the girth during the departure and serves two purposes: it maintains the forward impulsion that prevents the horse from slowing rather than transitioning upward, and it prevents the horse's barrel from falling to the inside and collapsing the bend during the departure. The inside leg at the girth is the energy-maintaining aid; the outside leg behind the girth is the lead-directing aid. Both are necessary, but they serve distinctly different functions in the departure.