A one rein stop is and emergency stopping aid.
You take one rein and haul the horses head around to your knee forcing them to turn and slow down. You will need to practice that, I do it every time I ride my horse so that when I really need it she knows what it means.
It's also good to help contol scared and onery horses.
Heres an artical over it.
Training ~ One-Rein Stop
My horse's ques are weird. I tried to work a que to make her turn with my legs but she justed wanted to run...