Running at a higher cadence makes the runner take shorter steps that land more underneath the body as opposed to longer, slower overstriding steps that land in front of the center of mass. The foot also tends to contact the ground more towards the middle or front. Here are a couple of pics:
Though it looks as though the runner in the top picture is overstriding a bit, his center of mass will be over the foot when it touches the ground. The bottom photo is me at about mile 20 of a marathon last May. You can see I am not about to heel strike, which is much different than the picture I posted last time. Many experts claim that foot strike is something that can’t be changed, and that some people will always be heel strikers. That is obviously not necessarily true.
When you run at a faster cadence, instead of reaching out with your foot and crashing on your heel, your feet kind of spin underneath your body. This is more efficient and requires less effort and straining. People who have a pronounced heel strike will find that it is less with a faster cadence. All of this adds up to a more efficient and less injury-prone runner.