I had actually copied & pasted this from his livespill a while back:
Types of Sled Movements
TYPE 1: moderate weight/slow movements... these are the best to improve nutrients uptake by the muscles... they thus facilitate recovery the most and help with hypertrophy. The also help with fat loss since more nutrients are sent to the muscles, and less to fat.
TYPE 2: light/moderate weight - fast speed: short bursts of 6-9 seconds with short rest intervals. Depending on the rest intervals these have a great metabolic cost. Since they combine both explosive and eccentric-less work, they have a profound effect on muscle insulin sensitivity. Combined with the metabolic cost, it makes it a GREAT fat loss tool.
TYPE 3: heavy weight/"just get there" kinda speed: I use very heavy 15m pushes/pulls with as much weight as I can do for that distance. I actually treat this as I would a strength lift and gradually ramp the weight up. I find this a great way to increase muscle mass. It is a great way to boost squat, deadlift and olympic lifting performance.