Although you can simply click on a square to remove it, dragging the mouse will leave behind a black line. Interestingly, quicker mouse movements will increase the size of the line, and slower movements minimize it. Larger lines help you clear multiple blocks simultaneously but, of course, avoiding red squares becomes more difficult. Several power-up squares will come in handy: purple squares add time to the clock, green squares provide bonus points and yellow squares replenish your health. Some levels also feature packs of flying red squares, which can damage you even while you're not drawing a line. The timer in the corner of the screen keeps track of the bonus points you'll receive after clearing the level. Take too long and it can fall below zero, actually subtracting points from your score!
Analysis: I would have welcomed a wider variety of hazards and more devious level design, but Valo is successful in its simplicity. The subtle soundtrack, generated by your actions, and the stark black squiggles left behind after clearing a level create a pleasant, distinctive atmosphere. While I found the default control style perfectly usable, a '2-click' mode is also available; in this mode you click once to activate line-drawing, and click again to turn it off.Valo also offers a randomize feature which, unfortunately, merely offers the same set levels as the normal game in a random order.
Cheers to Ian and Sergio for sending this one in!