I'll let McDougall explain what the 100-Up is.
In short, the 100-Up is a simple exercise (in both its Minor and Major components) that forces a person to run with proper form. Upper body straight, with a slight lean forward. Arms at side, swinging appropriately. Knee up, to hip height, and then foot back down, landing on your forefoot. Lather, rinse, repeat 50 times per leg. Eric Orton, McDougall's coach, teaches a similar drill - but simply calls it running in place (do watch the whole clip, but this drill starts at 3:10).
I'm doing the Minor, and if I can do it properly, the Major, every day. Why? Because I don't pick my knee up very high when I run - I do what many ultra runners do in the later stages of a race. I shuffle, keeping my feet close to the ground, not driving my knee up and forward. My high school CC coach, who crammed biomechanics and efficient running into our heads (although not necessarily our legs), would not look kindly on my form.
Neither exercise is a slouch, and both are difficult in their own respects. But they will teach you to pick up your knees, drive yourself forward, and take the longest stride you can for the speed you want to travel. The ability to take a longer stride will help when you need to run fast. Lets hope it works for me. Initial tests have put some serious strain on my quads and hip flexor muscles.