how does the islan made to fall or rise ? hat make it so?

Gulliver learns that Laputa is floating above Balnibarbi, the island on which he landed his canoe. Laputa contains 10,000 acres and is perfectly circular. It is able to move about the surface of Balnibarbi but not beyond its borders, and it can move up and down because of its magnetic forces. When a town from Balnibarbi acts up, the King has Laputa moved directly above it so that it can receive no sun or rain. No one from the Royal family is allowed to leave Laputa.