I am someone who uses contrast overspeed methods as described by Brent McFarlane and used at LSU once or twice a season:
(1) NONE of Glen Mills, Steve Francis, Charlie Francis or John Smith have said that they use overspeed methods (but it does appear that Jamaicans do use contrast methods, but these do not have to involve overspeed).
(2) The scientific literature does not seem to conclude that overspeed itself works, but contrast versions (more dangerous than straight overspeed) do have good results, at least if used sparingly.
(3) I read a paper last year suggesting that contrast methods work by causing UNEXPECTED microtrauma to eccentric muscle fibers. After recovery, this results in an increase in stretch-shortening strength and velocity. In other words, it’s a form of shock training, in Verkhoshansky terminology. BUT repeated use of the training method removes the unexpected part, and it doesn’t work any more.
I’ve seen significant effects for the contrast version after just 1 workout…but for the reasons in (3) above, it doesn’t work after more than about 2 sessions. So I would say that while the girl has seen a quick jump in performance, that improvement won’t continue–and every time you do it, the injury risk increases.