Shoulder-in is the easiest lateral movement to ride, and it is when the horse is bent around the rider’s inside leg and the forehand is 1 step only to the inside track. The horse is travelling in the opposite direction from the flexion of his head. If you stand in front of the horse, you see 3 legs. You see the inside fore on one track, the outside fore which is hiding the outside hind, and the outside hind.
Use it to warm your horse up so that you improve the weight carrying ability of the hind leg. The horse develops rhythm and the bend through his length and his shoulders become freer and his mouth more supple.
Ride Shoulder-in in walk and rising trot to start with and as the collection becomes easier in sitting trot. Start as you come out of the corner along the long side and when you are in sitting trot add small circles.