Transition from Trot to Canter on the Forehand
This transition movie (below) shows the change of gaits on the forehand. The leg swap not only lasts into the second stride after the canter is achieved, the canter itself has less ground covering scope as indicated by the positions of the stance legs behind.
The transition shows a lurch as Rio crosses the speed gap between trot (about 4-5 meters/second) and canter (about 6-7 m/s) on a front leg. His canter is a bit too quick to allow the dressage transition that is level, prompt and fluent, an example of the sensitivity of dressage transitions to tempo.
Front legs have a greater proportion of braking function than propulsive function. This plays a role in the balance of the transition itself and in the canter that follows. If you let the movie play through, then use the controller to step through individual frames at your own convenience, you can see how this unschooled version of a transition works.
The horse begins the leg swaps on the inside fore with the other three legs in the air. Although the withers are above the croup with apparent positive relative elevation, as measured against the grid of the fence, this is not true engagement. Engagement involves bending of the joints of the whole hind quarters PLUS positive relative elevation. If one rear end joint lags in its position (in this case the hinge sacro-lumbar joint), the whole hind quarter is placed differently and the landing of the first step of the canter (outside hind) will not be as far under the horse as in the dressage transition.
For comparison, examine this QuickTime movie of a true dressage transition performed by Raynyday Maximillian. It is from collected trot to collected canter and uses the same leg swap with a transient diagonal pair as the balanced transition (medium trot to medium canter) by Rio in the trio of images on the previous page.