I have been training for speed and explosiveness for a little bit now and I am enjoying myself immensely for a couple reasons. First, in general, I'm faster at all of my other lifts. I think this is largely due to CNS adaptation. When you train the clean, or the snatch with the right weight, only a full commitment by your CNS will do. I found this new speed particularly useful dead lifting. It's almost as if there is a better mind/body connection. Consequently, when I train at my 90% of maximum, I pull more reps. So I am stronger at least in theory.
Secondly, recovery seems faster so I can train more which means progress has been better in terms of weekly volume. Of course that could be my own training belief that volume is king manifesting itself within my psyche.
Additionally, training at the higher volume and speed has the additional benefit of leaning me out which is my current goal.
In the end, I don't know if the fibers are being retrained or not, but I certainly feel that more fibers are getting the message: "Pull this F*cking weight off the ground!" ;-)