To perform a double full, or any skill requiring additional height you need to stretch through your shoulders a little more into the back handspring which yields a sling shot effect that makes the feet snap behind the body for a forward lean set which creates a angular lift giving you the height you need to complete the skill. Body should be tight but stretched through the shoulders. At the time of your set into the double full you must stay totally tight, arms by ears with good posture and power base, core, thoracic area and head engaged to obtain maximum height for the double.
The double is accomplished by pulling after feet have risen to at least horizontal in the layout. The more lean on the take off the more twisting you can accomplish. The degree of tilt in the twisting shoulder produced in your pull also dictates the amount of twisting.