Originally Posted by Chiilaa
Change the direction you are going. You are on the right rein if your horse's right shoulder is to the inside, left rein if the left shoulder is. To "change the rein" one simply switches direction so the opposite shoulder is to the inside. Crossing the diagonal is probably the most common way that people change the rein.
Here I was thinking it was something complicated
Geeze, they could have just said change direction lol