As described in “Get Tough! How to Win in Hand-to-Hand Fighting, as Taught to the British Commandos and US Armed Forces” by Major W.E. Fairbairn (1979):
*Your opponent is lying on the ground.
Take a flying jump at your opponent, drawing your feet up by bending your knees, at the same time keeping your feet close together.
When your feet are approximately eight inches above your opponent’s body, shoot your legs out straight, driving both of your boots into his body, and smash him.
Note: It is almost impossible for your opponent to parry a kick made in this manner, and, in addition, it immediately puts him on the defensive, leaving him only the alternative of rolling away from you in an attempt to escape. Further, although he may attempt to protect his body with his arms, the weight of your body (say 150 pounds), plus the impetus of your flying jump (say another 150 pounds), will drive your heels into your opponent’s body with such terrific force that you will almost certainly kill him. Steel heel-plates on your boots will make his attack even more effective. *
Practice this kick on a dummy figure on on the grass.