They're very like draw reins, except that they are fixed to the saddle and don't run back to the hand. That eliminates the unfortunate leverage effect of draw reins which so often teaches the horse to duck behind the bit unless they're checked up.
Here's a couple of photos of Kody working in his sliding side reins this spring over cavaletti.
As you can see, I adjust my homemade set by tightening or loosening the knot on one side. Not very high tech, but it works!
This is the kind of stretching I'm trying to encourage. He isn't engaging in this photo, but I like the way the muscles in his neck are working all the way back to the wither.
This is Kody back in June of 2006 when he first was introduced to sliding side reins. Not working well:
Figuring out how to stretch down as he races off the bottom of the hill:
See how he went from using the bulging muscles of his underline to using the ones on the top of his neck? Over the next year he learned to bend and find his balance somewhere in the middle instead of rooting around near his feet and being stiff behind the saddle, but he still had stifle problems at this point and lunging (especially downhill like that) was very difficult for him. You want to keep your sidereins low until the horse finds his balance, then you can start attaching them higher on the surcingle depending on the horse's conformation and what you're trying to teach him. They should be loose enough that the green horse can stretch comfortably and in fact may only catch him when he goes to raise his head and hollow out. When he's built up enough muscle you can slowly tighten them until he's on the bit all the time like Kody is in those first two photos. Keep your sessions short at that point though, side reins do not allow the horse to have a stretching break like a driver's hands will. Warm him up, put on the side reins to accomplish a specific purpose, then take them off and let the horse stretch. Only keep them on if they're so loose that they only prevent throwing the head way up.
Leia