I think Speedster is thinking of using sleds to improve specific strength or strength endurance in the ense of asking the athlete to deliver good technique while carrying some fatigue from an earlier depletion run, then re-loading with back-up runs pulling a light sled.
If that’s what you’re thinking, it could work but you’d better be very clear with the athlete about the objectives of this kind of sled session because there is a risk of injury. Then again, if you keep the sled very light (maybe even unloaded) and ask for a controlled effort gradually building up speed and into a tall, upright posture I think there is plenty of merit in the concept.
The only sled work I did was 1x60m sled; 1 x 80 alternate high skips (take-off every third step) over about 80m; and 1 x80m-100m controlled sprinting buildup. That was one set (not necessarily in that order) and we often did two reps of each activity. Then we’d do from 4 to 6 sets for the session. It fell in the speed-power second two-weeks of the 6-weeks general prep phase.