Susanne..
are you asking how to keep the extended trot, or collecting him into an animated collected trot?
The key is the motor..the rear end MUST be engaged, while the horse moves into the bridle (slight brakes)...here is a copy of my "Tips" sheet
Collection and Impulsion are the most important factors to teach a horse, after the whoa command! Long-lining has the same principles of the bitting rig, but with the added benefit of steering. In long-lines the horse can be taught all their gaits, head position and impulsion.
True collection starts from the hindquarters of a horse through engagement, or the motion of a horse "sticking his tail in the dirt". The horse learns collection and impulsion through the “go forward†signal combined subtle finger pressure on the lines. When your horse starts off at a walk, and at his first step willingly moves his entire body into your fingers, releases his poll, and "tucks his nose", you will have achieved the first step towards ultimate collection. "How far can you take this response?"
The walk is one of the most important gaits to work on. By using half-halts and clucking (think of using a brake and gas peddle of a car) you can start teaching the horse impulsion. This exercise cannot be overdone, it’s much more difficult for your horse to maintain an active, correct walk, and rounded through the top-line, than it is to walk along at a “swinging†gait with the neck not flexed. This exercise will also help develop the muscles of the horse’s neck, back and hind end.
To encourage the horse to really utilize the motor (rear end), I keep the line down around the back end-just above his hocks. The line creates a “barrier†so to speak, with the outside curve of the horse’s body being “held†by the line. With the line above the hocks, it seems to “push†the rear into the bridle. I use draw lines on all the horses. The critical key to draw lines is ensuring they run smoothly and easily! Any tendency to drag or snag will cause the horse discomfort and defeat the “light and airy†idea. With draw lines, I do not need lots of pressure to get the horse to tip his nose in. With the “hangers†you get an extra “draw†along with keeping the lines low on the horses body.
If the horse is used to a check, and is well broke to long-lines, I gradually raise his head, teaching him to collect himself and to carry his head perpendicular to the ground. Keep in mind that very few miniatures are built with their necks rising perpendicular out of their shoulders. Most miniatures’ necks are set on somewhat lower. If they’re checked up too high, they hollow out their backs and lose cadence behind, or swing from side to side. While the horse is checked, I will raise the “hangers†so the drawline is relatively higher.
The most important thing the check does is to teach the horse to raise his front end, causing him to drive off his back end: to come strongly off his hocks. Horses naturally carry 60 percent of their body weight in the forehand. In order to appear light and ‘airy,’ the horse must learn collection so that he doesn't appear heavy in the forehand.
As the horse becomes more programmed and conditioned to walking in a collected frame with impulsion, he can be asked to move out at the trot. Learning how to collect your horse’s forward energy on the bridle at the walk will make the transition into the trot much easier.
The secret here is persistence, and consistency, on the part of the driver.
Kim