The 100-Up consists of two parts. For the “Minor,” you stand with both feet on the targets and your arms cocked in running position. “Now raise one knee to the height of the hip,” George writes, “bring the foot back and down again to its original position, touching the line lightly with the ball of the foot, and repeat with the other leg.”Here is Christopher McDougall showing how it's done:
That’s all there is to it. But it’s not so easy to hit your marks 100 times in a row while maintaining balance and proper knee height. Once you can, it’s on to the Major: “The body must be balanced on the ball of the foot, the heels being clear of the ground and the head and body being tilted very slightly forward. . . . Now, spring from the toe, bringing the knee to the level of the hip. . . . Repeat with the other leg and continue raising and lowering the legs alternately. This action is exactly that of running.”
He then published two blog posts related to injuries, posture and the 100-up exercise: the barefoot MD, in his own words, 100-up and the vampire bite (aka, plantar fasciitis).
Some interesting comments on McDougall's post: