i think we are confusing 2 different topics. You are talking about the quality of lope departures from a reining perspective, on which I actually agree with what you're saying.
I was questioning why holding them back while spurring or over/undering would increase their sensitivity/response time moreso than spurring or over/undering without holding them back - purely relating to getting the horse to move more forward. Regardless of the quality from a reining perspective.
I use spurs to lift my horse and for lateral movement, I don't use spurs for speed. If I roll both my spurs on my horse while loping I essentially slow him down because I lift his rib cage and shorten his stride. I bump with my LEG, shift my seat and offer him the reins to speed up. But spurs come after a shift in seat. Always seat, legs, hands, spurs. Some very very good horses will change direction just with the turn of your head.
If I am entering a run down AT A LOPE and my horse doesn't speed up when I cue him, THEN I will over and under and increase my speed to the end of the arena, pick up and slow the lope around the short end and reenter the run down down the long side again or diagonally across the arena, WHEN the horse is picking up speed fluidly and in a CONTROLLED way and is travelling straight, round and soft (solely from my cluck and seat cues) then I ask for a stop (slide) I ALLOW THE HORSE TO STOP, therefore creating a desire in him to travel correctly and pick up speed into the run down without creating anticipation. He doesn't stop until he is travelling correctly. Again nothing to do with a lope departure.
The desire to run down correctly also has nothing to do with the horse wanting to slide either. The quality of your run down however does predict how correctly your horse is going to slide though, again just ONE part of the lope, speed control - and has nothing to do with the lope cue. Once I ask my horse to lope everything is released, I don't need to keep a leg on him, I only put a leg on him to change speed. He will keep loping until I say do something different.