Their goal is to find jobs and managers with low expectations, then sandbag as much as possible ("Gee, this task is harder than I thought. Going to take a couple weeks longer than we estimated!").
Had a team member try this and an old company. We caught on quickly when they couldn't keep up with their workload and were constantly unavailable during the day. Really sucked for the rest of the team who had to pick up the slack this person created by pretending to work full time.