How often have you heard that term? Sure, it sounds like a pretty simple concept until you try it. Here's a detailed breakdown and a "cheat" exercise (that's not really a cheat at all).
Aids for a circle
Baldwin Stables
How to get inside leg to outside rein connection
Ehlen Horse and Tree Farm
Michael Peters Dressage
Inside Leg to Outside Rein - What this means and an exercise to finally FEEL it - HorseClass
Bucking Horse - Stop Your Horse Bucking in Canter
Lighten Your Horse's Forehand – From The Hind End – Horse Listening
horses – Horse Listening
How does a rider control the speed of a horse? - Quora