Think about trying to maintain your balance if you lift your toes off the ground and just use your heels. This is unstable and will have you wobbling around. Now what if you lift your heels off the ground and balance on the front of your feet. Much more stable. For flat surfaces, running on your heels doesn't make a huge difference in stability (i.e. you probably won't fall down). The amount of time your heel actually spends on the ground is small and there are generally no disturbances that would affect your stability in this short amount of time. However, if you run on an off-road trail where there is uneven terrain and obstacles such as rocks and roots, there are many opportunities for disturbing the stability of your heel-balancing act.
If you had to take a guess at which part of your foot would more effectively transmit shock to your knee would you say heel or toes? Well your heel has a straight line of sight to your knee. For your toes to do the same thing your foot would have to be vertical, which as far as I know, is only done in ballet? Now think about what you would do if you were running barefoot, or maybe try it. Your body wouldn't, in a million years think to run on the heels of its feet. The reason we run on the heels of our feet in shoes is that there's a giant wedge under there that makes it unavoidable. Then you might ask, well if there's a giant cushy wedge there, why should I worry? Well, the giant cushy wedge absorbs SOME of the shock but not all of it. When you run on the front of your feet you absorb shock with your leg muscles. You want your legs to be like springs, not like hammers.
So the only thing left to talk about is experience. I've tried it out for a few days now. I bought some New Balance WR790's which have very thin soles for trail shoes. Most importantly, they have a very low heel, which makes it easier to practice this style of running. (You're gonna have a hard time running on the front of your feet with big heels unless you're going uphill.)
Observations So far
- Less ankle rolling (stability)
- Tired leg muscles (shock absorption)
- Harder to maintain running style when tired. Heels want to go down.
- Takes time to build up speed and distance since you're using muscles more. (You're not gonna be a rockstar runner overnight.)