You tie a thread on your finger before jumping in the pool to remind yourself of the promise you made to your mother. The thread represents the promise you made to mow the lawn after you finished swimming in the pool.
After you get out of the pool, you keep the promise you made to your mother. So you get on your yak (やく) powered mower and trim the grass. Yes, your mower is pulled by a yak. It’s very efficient.