Jane Savoie is asked questions about riding all the time and this reader wanted to know where to place her leg for the canter depart, and when to release the aide. 

Swing your outside leg back ONCE, and then bring it back to it's normal position on the girth. Think of it as a spring-loaded action or a windshield-wiper like action. If you wait for your horse to answer, he's not listening to your leg aid. If he doesn't canter right away, give him a little bump with your outside leg 
or tap him with the whip. (Carry your whip in the outside hand for the canter work so you can use it to reinforce your outside leg aid.) As soon as he does the canter depart, reward him. 

You can teach him to canter by holding your outside leg back, but when you get to the point of doing half passes in the trot, your horse will be confused. He won't know whether to stay in the trot and go sideways for a half pass or to pick up the canter.

It's easier to teach him to canter from an aid that ONLY means canter depart than to teach him to canter from that aid and then have to reschool him when you get to trot half passes.

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