How to use:
You use a crab trap by crafting it then while holding it, right click on water and you will throw it into the water. It can be left unattended, but it is not reccomended because the crab could steal your bait and escape.
Once you catch a crab:
Your Crab Trap will be unbaited. You can rebait it by crafting it with another raw porkchop above it.
How to know when a crab is trapped:
When a crab is caught, your crab trap will move. When it is moving, that is when you should pull it out. If you pull it out when it stops moving, the crab would have escaped and your trap will be unbaited.