Never done it, but heard some popping noises coming from new LR and read up on it. Turned out to be my Trigger in my other tank across the room, but anyway I read a lot about mantis shrimp. Says to locate the residence of the shrimp, be patient and wait for it to go into its cavity (hole). Once in the rock remove the rock and put in a bucket of saltwater. Pick up rock and using a turkey baster or syringe, squirt fresh water into the hole. The shrimp should come flying out of the hole, or eventually come out after many squirts. If that does not work do everything the same, but use carbonated water or club soda instead. The article said to only use a freshwater dip if you are not concerned about losing any other marine life that my be living on the rock.
Good luck!