"Crab backs" for bait

I read references lately to "crab backs" as a bait here, here, and here for example.

Does this mean the top half of a crab shell, plus whatever sticks to it? Crabs with their legs cut off? Something else? I assume we're talking small crabs like shore crabs or (at the most) kelp crabs.

Thanks a ton.
Also @jcool3 - do you use the crab innards before or after cooking? Or does it matter? Any tips for keeping it fresh?

I tend to catch a LOT of crab and I don't eat crab butter so I just chuck everything that comes out of the shell usually. Happy to try using this stuff as bait if it works!