BILL: First of all, before you worry about the aids, you have to visualize what the movement being called for looks like. If it all sounds like technical gibberish, substitute the words “upon the” or “around the” for “on.” In other words, turn on the haunches equals turn the rest of the horse around the haunches. That ought to make your task more clear.
Then, as I’ve proposed before, [See About Recipes—and the Queens in DRESSAGE Unscrambled], if you can picture how the movement works and how the horse is supposed to be positioned during it, by applying a little logic the aids should practically suggest themselves. For example, if the horse is supposed to be bent to the left, there’s only one combination of leg placements which will produce that bend.
As for the two exercises in question, I have a pair of images to differentiate between them.
For turn on the forehand, I picture a tidal river with a sailboat in the channel moored to a red buoy. When the tide is going out, the current makes the bow point upstream towards the buoy. When the tide changes, the bow continues to face the buoy as the stern swings around the mooring till the boat points downstream. In turn on the forehand, keep the horse’s chest pointing at the “buoy” as its hindquarters swing around it.
Turn on the haunches is an entirely different image. Here think of your horse as the hand of a clock. His tail is at the center of the clock’s face. His head is at twelve o’clock. To make the turn, the tail must remain at that center point while (making a turn on the haunches to the left) the horse’s head passes by eleven, ten, nine, and so on.
If you can do that and remember that turn on the forehand is a variation of leg yielding in that the horse looks a little opposite the way he’s being displaced, you’ve got that movement nailed. As for turn on the haunches, it’s a lower level version of a pirouette. Can you visualize a canter pirouette with the quarters going around the forehand? Not without your horse falling over! So picture a canter pirouette—the horse staying bent around the inner leg and turning in the direction of his lead. In turn on the haunches at the walk, the positioning and direction of movement is identical.