The heated underblanket is placed on the mattress under the fitted sheet. Switch it on half an hour before going to bed and you have a wonderfully warm bed.

I have the heated underblanket on the mattress and the fitted sheet over it. It works great. I would never give it away again.......

It also works without the app. Manually you can only: 1.) set the two zones individually from 0 - 9. 2.) Set the heating time from 1 to 12 hours. The app is simple and highly recommended because you can then set heating periods in a calendar. So for example, from 19:00-20:00 preheat level 9, then from 20:01 to 24:00 on level 4, then between 00:01 to 08:00 level 2 or 3. Incredibly practical. We've been using it for a 5 year old who was kicking the blanket off and therefore kept catching colds. Now he sleeps nice and snug.