This is something which the Canadian edition is in the middle of trying out, which might mean it's also a field test before bringing it to the States. Because we all know Alison Grodner is the kind of careful producer who likes to make sure a twist will work before bringing it to the central product.
Basically, this is BB's short-term hidden immunity idol. The contestant who discovered it had to work out a series of riddles which eventually led to a secret room and, after a lot of searching, the SV itself.
1. As with a normal Veto, it can be used once to remove someone from the block, which does not have to be the holder. If you save someone else, you can't be nominated in their place. For the States, HoH would presumably pick the replacement nominee.
2. It can be played after the regular Veto, and you can presumably hold both. However, you can't pull it out just before the vote: it has to be used during the actual Veto ceremony.
3. The SV remains active for three cycles. So a week-in-an-hour would take one out.
4. Prior to actually using it, if you tell anyone about the SV or the room it was found in, you are automatically evicted.
So this little rat manipulation tool may be on the way to California in a few weeks. Prepare.
Thanks to the ex-Worsters for the Incoming Stupid alert.