I guess I'll step up and play "some idiot". ----In order to heal strike (which, apparently more than 75% of all runners do, according to Steve Magness) your impact point of your foot hitting the ground will be slightly in front of your center of balance, just like when you are walking. When you toe or mid strike, the strike point is closer to directly under you. The difference is that the impact point of heel strikers has a slight decelerating effect at the moment of contact, each stride. Toe strikers "slap" the ground directly under them as they swing their leg through. Also, their foot is hitting the ground "pre-sprung", unlike heel strikers, who must use more calf muscle energy to get the same spring. Also, also, the time that a heel strikers foot stays on the ground during the stride is significantly longer than toe strikers. ---Most of the fastest runners in T&F are toe strikers. Some heal strikers have been just as fast, but there are not many elite T&F heel strikers. Heel strikers definitely make up most of the ranks in road racing however. ----Longer distances make even toe strikers morph to heel strikers as a race or run distance goes longer.
Professor Jumbo J, why do think that heel striking is less impact? a heel strike does not allow the calf muscle to come into play during impact. The talus, knees, hips, and back take it all with the calf not even firing on impact.