Desperately snatching an improvised weapon from the surrounding area whilst fighting.
The object grabbed will be something completely random, previously dropped firearms and the like would not count.
In the same way, if the fighter can calmly saunter over to a rack of pool cues or something then that isn't in the spirit of the category.
We are talking last gasp, desperation tactics where the person literally grabs anything to hand without any pre-conceived strategy.
The classic Grab Bag scene might be: The heroine is being choked to death but the camera cuts to her flailing hand, which is tantalisingly close to a conveniently small but heavy vase. If she could only reach it she could smack the baddy over the head! After several seconds of edge-of-the-seat suspense she manages to reach it and it is as if she read our mind, as she does indeed whack him - what a stroke of original brilliance!
[note for geeks]The title for this category is an homage to "Scorched Tanks" on the Amiga, where you could buy a relatively cheap "Grab Bag" in the hope that it would turn out to be a nuke or something equally devastating - but it invariably turned out to be something completely mince when it was launched[/note]