The quality of transitions is one of the indicators of the degree of "connection" and "throughness" the rider has achieved. Transitions have always been important, but the 1995 national tests began to stress that importance by giving transitions their own score. No longer could a rider have a nice extension and a poor transition and end up with a good score by default.
When your horse is connected, it moves as one unit. The rear end doesn't come along just because it's attached. It actually carries and pushes the front half of the horse. When a half-halt is given, it travels all the way to the hind fetlock, demonstrating "throughness." The horse should show no resistance in the lower jaw, poll (or upper jaw), neck, back, or hind leg. No half-halt is complete until you give, and the half-halt has not come through unless the horse demonstrates a few moments of self-carriage when you do give.
Upward transitions should be preceded by a successful half-halt. During the resulting moment of self-carriage, the horse can be asked to lengthen within a pace (and not get quick or run) or change paces entirely.
Downward transitions are often where problems with "throughness" show up the most. We see either resistance (the horse comes above the aids) or no real transition at all as the horse falls on the forehand, leans in the hand, and slows down because of an upcoming corner rather than collects. Collected paces are shorter and more animated, not slower. If you end up pulling or being pulled, instead of pushing your horse through a corner with a soft inside rein, not only has your half-halt not been successful, but also you are not realizing the benefits afforded by riding proper corners.
Downward transitions require more driving aids than upward transitions to facilitate shifting the balance more to the hindquarters, which lower to achieve the relative elevation of the forehand. Balance is required to achieve soft--not harsh, jarring, or abrupt--downward transitions.
Everyone talks about and supposedly rides half-halts, but it is the transitions--those between movements, between paces, and within paces--that present the clearest and most unavoidable evidence, to those willing to seek and understand it, of whether or not those half-halts (and therefore transitions) are successful.
-- by Suzanne May