You have a limited amount of time to line up the colored blocks as they fall from the upper part of the screen. But the problem is, as you destroy blocks and gain points, the direction from which the blocks fall will change.
So although the game may start with the blocks falling from the upper part of the screen, it'll only be a few seconds before the direction changes and you have to place them as they arrive from the left or right side of the screen.
Direction changes aren't the only factors that affect gameplay in 6x64. If you manage to connect more than three blocks together, you'll get a multiplier.
6x64 is a complex but fun puzzle game. The best part about it is that it has a good amount of achievements, and an online classification board so you can compare your high scores with other players from all over the world.