You can use it, either as a re-roll or to stop a victim.
I works like this: When your opponent moves a victim you can, at any given time, scream: "STOP!" And throw the card at him, he then has to stop the victims movement in that space. (This is how it is written any way)

In practise you might want to wait 'till he finishes the victims movement, then play the card and tell your opponent which space (the victim moved through) that you want it to stop in.

So basically you can stop a victim on any space it has moved to during the suspense phase.

use the re-roll