He claimed they had an entire physics class on this very thing. I assume the response came from that class but I really don't know for certain.
"Since someone mentioned physics, the physics are basically that your center of gravity is moving at a steady rate (horizontally), so by laying out, you make yourself 'longer', getting your arms or legs to the bag before your CG gets there. Running through (upright), with your CG about in your middle, means you get there slightly later.
If you slide, actually hitting the dirt before you get to the bag, then your CG is no longer moving at a constant horizontal rate (you're slowing down from friction), so you would need to lay out in the air, hitting the dirt and the bag at the same time, with your finger tips."