As you ride, your legs should remain in close contact with your horse's sides at all times. When your horse is scared of something and is shying or bolting, it is important to keep your legs snuggly wrapped around your horse. If you too become anxious and scared and squeeze too hard, you will send your horse forward more than you want to. If you push your heels down and away from your horse, losing the contact between your calf and the horse's side, you will be sitting in a way that will make the horse want to drop his back and raise his head in an uncomfortable and nervous position. When you keep your leg contact on, it acts like a big hug to make your horse feel secure and puts you in better position to use all your other aids more effectively.
The same is true of an over anxious horse that wants to trot or run instead of walk. Taking your leg off and just shorting the rein and pulling back is not completely effective as the horse will just do a prancy little gait called a jig. Sometimes they will jig sideways as we try to restrain them. Sometimes they will even start to rear up. A soft and effective use of leg (and of course seat) helps keep the horse's back rounded up and his head down. In this position he is "on the aids" and much more responsive to us.
Though it may sound contradictory, the proper use of our legs will also make our horse stop better. Again, because the horse is "on the aids" when we are using our leg aids correctly, he will be much more responsive to our seat and hands when we ask him to halt.
