What is a shoe 'drop' for?

It is the gradient of a running shoe. It was apparently originally introduced by Nike for their track athletes who were getting over-use injuries in the Achilles tendon. This allowed them to train more (when they should have rested). There seems to be no valid physical reason for it to be there and it is abnormal but it has been adopted widely and is very bad for the skeleton as, even walking, it stresses the knees, back, neck - it bends the body. In running it makes it impossible to land the foot under the hips and therefore greatly increases impact forces and injuries. It leaves the running in a squat position and removes the possibility of using the natural elasticity in the legs to 'bounce' off the ground as a good runner does.