Obviously, your left hand controls your bridle through the bridle reins, which controls your horses front end and the regulation of your forward movement. Your feet control your horses body when it comes to both lateral, side-to-side movement and also pushing your horse forward. When it comes to being able to get into position with your horse, those are the controls that give you the ability to pinpoint your shot. How you use them is going to make all the difference.
Every horse is different, but for a heel horse you want your horse to arc and bend, with his nose tucked in a little bit to the left and his ribs in an arc. You want his whole body kind of bent in a left arc. Applying pressure with your foot to his left rib cage and keeping your left rein a little shorter than the right rein promotes that bend and arc, which is going to set the horse up to hold his left shoulder up around the corner. That will prevent him from cutting the corner. That left arc also positions a horse's head, neck and body to make a good left turn.
Initially, when working with a horse to get him shaped and framed up the way you want him, you have to be careful that his hips don't slide to the right too much out of position. If they are trying to slide out of position, you have to use your right foot toward the back of the rib cage to keep his hind quarters over to the left, while still maintaining that arc.
Using your feet and hands in a consistent manner, developing a pattern and giving your horse time to develop and learn that pattern gets the best results. Don't expect your horse to figure out what you're trying to make him do in one day.
Having your horse picked up lightly and having the calves of your legs pushed against him lightly keeps your controlling parts in a position where your horse is able to feel them at a moment's notice and there aren't any surprises when you go to use them to make a command.
Being able to work with a horse's natural ability to cow, and also being aware of how he's responding to your controls, is another important part to try to blend together equally. For instance, if I over-control my horse I can completely take the cowing instinct out of play. That's something I don't want to do, because if I override his instincts to work then I have to do it all myself.
How I'm controlling my horse-and how he's reacting to how I'm controlling him-must be constantly monitored. It's a part of my roping I always pay very close attention to. Sometimes I find myself telling him to do something with my left hand or feet that's making him make an error. But when I really analyze it, nine times out of 10 when he makes a mistake, it's my fault.