Powering down converts your HP into HIVE and you will be receiving 1/13th of your available HP every week.
You'll choose what percentage of your available HP you want to power down. Available HP is your total HP minus your delegated HP.
For example, you'll be powering down 100% of your available HP. Each week starting from a week after you executed the power down, you'll be receiving HIVE equivalent to 1/13th of your total available HP. This will continue for 13 weeks (converting all your starting available HP into HIVE) or until you stop the power down.
I hope I explained it well.