Crab lining involves dropping a weighted bait bag into the water and keeping an eye on it until it starts moving. Sometimes the bait bag can be inside a net, but either way, the bait is on the end of a string, no rod involved.
When you think you have a crab biting, pull it up.

With them being sea crabs, it's good to have a big bucket of sea water (tap water will kill them) next to you to put the crabs in while you put the bait back into the water. I'm sure I caught the biggest one, big enough to eat if I'd known how to cook them, but it let go of the bag as soon as I pulled it out of the water.
It was about this big.