Say you're running down hill and start to lose your balance. You run faster and bit more awkwardly right? What do you do? You slow down for a moment, catch your balance by bringing your upper body more upright and resume running down a hill a little slower and in balance. You just half halted yourself.
It's the same concept on your horse. The purpose is to rebalance your horse. You're trotting along, he starts to get strung out. You sit tall, close your legs around his sides asking him to step under himself more while using your hands to contain the energy momentarily in the outside rein.
That's basically it.
You just have to see your distance...you don't have to like it.