I'm not sure if it can used to retain (ie, extend) status (that you already have), but it can definitely be used to regain status you just lost.

This past April 1 I dropped from Diamond (which I'd gotten a couple years ago from some other promo) to Silver. I immediately singed up from the MVP Fast Track, and became Gold (per the website) in fact after just 3 (rather than 4) stays, though since by that time I already had the 4th stay booked, I did that 4th stay just to be sure.

The only part that's unclear (since it's so hard to get accurate "when does my status expire" info out of HHonors, especially online) is whether you can use it before April 1 (while you're still Gold or Diamond, but knowing you'll expire to Silver or no status on April 1 because you had way too few stays/nights the previous qualification year). If you try it before you lose your status, you won't know if you've succeeded for a while, and then you won't know your true status expiration date for even longer:

http://www.flyertalk.com/forum/hilto...e-hh-card.html

In my case, I researched MVP Fast Track To Gold before April 1, but waited until after April 1 to have my 4 stays, and then did them very quickly (hotel hopping at 2 hotels!) because I needed the Gold for a stay in early May! And, btw, I did those 4 stays all at (USA) Hamptons, where everyone gets free internet and free breakfast anyway, so I didn't have to feel the "pain" of my temporary Silver status.