It is the centered position of your body in the saddle over the middle of your horse with equal pressure on your seat bones and buttocks. Your back is straight and flat, shoulders, hips and heels fall in a straight line, your head is up and your arms are at your sides comfortably with elbows slightly bent.
Nothing is strained or tense.
You must be relaxed enough to follow the rhythm of the gait, being neither behind or in front of the motion. Your ankle joint, knee, hip and waist allowing you to keep your upper body still. The legs have their influence by yielding to produce and maintain motion, forward and upward or changing gait. They also function to resist, or act to engage a horses hind legs. This means to encourage him to drive his back legs up under himself to give lift and momentum to a gait.