A sports car driver directly influences the outcome of the car's performance. When your vehicle has four legs instead of four wheels, you are in the driver's seat and determine the outcome of the ride.
Using your rein, leg and seat aids correctly creates the conditions necessary for getting your horse's rear in gear. Apply them incorrectly, and you will hinder your own efforts. Success starts with soft hands. To give your horse the support he needs without restricting his forward motion and ability to adjust and reposition his body, your hands and arms need to be free of tension. Be aware of any tightness that creeps in from your shoulders to your fingertips. Carry the reins lightly in your fingers, never pulling, jerking or holding them tighter than is necessary to keep them from slipping.
You can help your horse even more by stretching tall through your ribcage and sitting in the center of your saddle. Relaxed hips and a soft lower back will help you move fluidly with your horse and not interfere with his movement, making it easier for him to maintain impulsion from behind.
Further Reading
Bodybuilding for the Western Horse
A balanced approach to western equitation