Neodymium magnets are the strongest permanent magnets there are. One the size of a coin holds several kilos.
The magnetic system
Each frame carries neodymium magnets on its back that lock onto a steel rail on the wall. Pieces go on and come off in seconds, with no tools and no holes.
That's why the wall can change so often: rearrange the pieces, rotate artists each month, mount and dismount without leaving a mark.