From bikeradar.com, Aug 7th, 2014:
The attack position is fundamental aspect of mountain biking. So getting it right will reward the rider with a more comfortable and controlled ride. Get it wrong and you run the risk a chiropractor visit or maybe even a free ride in an air ambulance.
In the video, Sam from Bristol based skills coaching company Pedal Progression talks you through the correct body position. A good position takes the weight off your hands, meaning there is less tension in the upper body. This tension restricts the movement of the bike under you and will prevent the ability to counter act sudden trail obstacles. Sam explains how dipping your heels prevents your weight being shifted forward and forces it down through your legs. He also offers some useful points that suggest why leaning back over steep terrain is not the best technique.