I read this online not too long ago and it was too good not to pass along:
"Someone once told me that your inside leg is like the wind
and your outside rein is the sail,
wind blows into sail and that sends the boat forward."
~ Nancy Allen
~ Nancy Allen
But what about driving where no inside leg is available?
I highly recommend Muffy Seaton's book and video: "Bending the Driving Horse". She explains it very well and shows it very plainly in the video. Once you learn this, you will use it with every horse, every drive forever. Worth every penny!