You “land” on your heel and roll to the ball of the foot, with a load of just your body weight for every step.
You land on the ball of the foot first and then put down the heel. Because of the swifter movement, you’re now hitting the ground with two times your body weight. And because of this added weight on each footfall, you have to be careful and make sure you’re doing this right.
You land on the ball of the foot, and there is no heel touching the ground. Now it’s three times your body weight and because it is faster there is no time for the heel to come down and the elastic recoil takes care of the forward push.
All other combinations are just non-efficient mutations. Like the one where humans combine the heel to toe form (from walking) with two body weights (from running)… that’s what Ron Burgundy would call “yogging”. Don’t do it!