'Swap' blocks to make matching rows or columns of blocks. Use the arrow keys ( or w,a,s,d ) to move the swapper and press Space to switch them. The aim of the game is to get the highest score possible before the blocks reach the top.
If you've cleared the screen, manually shift the blocks up with Ctrl.
For bonus points, match more than 3 at a time. Also, create Chain Combos for a huge bonus by making a chain reaction!