When sand falls onto a slab, it will normally turn into an item. However, when you put a piston below the slab, sand just falls on the slab, piling together.
Then, if you push the sand, it expands.
If you block the way, sand will not expand, but if you stand on the sand, you will be pushed far away as if the sand has expanded.(Like the stairs with a piston in the picture below)
Someone even built an ultra-fast transportation system with it!
However, it randomly crashes the game.
(Other blocks like gravel acts the same)
Platform: Windows 10 UWP, Android, IOS