I’m not so sure. Playing beach volleyball on sand does not translate into increasing my hard-court vertical leap. The surface ‘gives’ somewhat and teaches skills mainly exclusive to that particular surface. I suspect the very specialized reactive processes that are happening to a sprinter’s legs on a hard track are also fairly specific to the surface. I can’t run as fast on grass, therefore it’s not teaching my nervous system to achieve heights it never has before. Recovery, yes, I get that. But to get faster? Besides the relaxation angle, which I can hopefully learn on the track anyway, I can’t see how. Just a thought!
I wouldn’t equate working on grass with sand, where there is virtually no return from the surface. Working on grass, like a slight hill or other resistive activities, creates some additional load, while, at the same time, offering some protection to the tendons from the sudden release characteristics of the very hard tracks. It may take a bit of time to re-adapt to the track, depending on the time you spend away from it, but we always found it useful for training camps, away from the competition period- usually no more than 2 weeks at a time.
Makes sense to me!