Posture and mechanics.
Could you be more specific? What postural/mechanical changes are occuring with a 20% sled that don’t occur with a 10% sled?
Football is about applying force quickly, at some point, you will be slowing the athlete down with too much weight as well. I wouldn’t think you would want to advocate HUGE pushes for a footballer anyway because they need to be able to cut. So they would need to actually alter mechanics in order to stay grounded. Football is a different than track in many aspects for that reason.
The postural and mechanical deviations that occur during a ‘cut’ far exceed the differences that occur by adding a little weight to a straight line pull with a sled, wouldn’t you agree? I don’t worry about slowing the athlete down a little…all resistance exercises are slower than sprinting. Like CF used to say, work all parts of the curve.
If we’re talking just track and field I wouldn’t argue, but I believe that the first step or two are much, much more important in sports like football and basketball than they are in sprints, especially anything over 100m, so extra work on the explosive start is critically important, IMHO.