"It turns out your center of gravity is where the momentum is," Peters says. "This is found half way from the tips of your fingers to the tips of your toes. In the headfirst slide, the center of gravity is lower than halfway between your feet and hands, so your feet don't get there as fast. It's faster head-first."
For a long time — until roughly the Pete Rose era of the '60s and '70s — players shunned the headfirst slide to protect their hands and faces. Spikes, evoking the Ty Cobb days, were weapons on the diamond. In the past few decades, players who prefer the head-first slide have taken to running while holding onto their batter's gloves to prevent their hands from opening up and being exposed to injury. While the percentage of players who slide one way or the other is not actually known, Peters estimates it's about 50-50.