When it comes to sprinters and hurdlers, and power athletes in general, they live and “die” by their ability to accelerate as well as their top end speed. According to Bruce Kelly, the power sled is a good tool for enhancing acceleration and by that I mean the first 5-10 meters of a race, after that it loses some of it’s effectiveness.
What kind of load should one use? Mr. Kelly posits that they should start with the standard recommendation of 10% of bodyweight and adjust from there depending on the ability, strength, and experience of your athletes. The key to look for, I believe, is how closely the sled allows the athlete to maintain close to normal acceleration mechanics. If it looks like they’re towing a small semi (i.e. hands on ground, crawling along) then it is probably too heavy. On the other hand if the load is too light the specific ground reaction force training you’re trying to develop may not happen.