You play Zerosum on a grid of blocks with numbers on them. Your goal is to make matches, by lining up at least three of the same number in either a row or column. Everything is done with the mouse. Select one of the numbers on the grid, and move it into an adjacent number. The two digits will add together. If the sum is greater than 9, the tens digit will be dropped, so 7 + 8 results in 5, for example. A block with a zero is a powerful wildcard—it matches with anything.
On the right is a list of the upcoming digits, which appear in the vacant space whenever you make a move. When you run out of these, your game is over. To prevent this you have to keep making matches, reducing the Extend meter on the left. Each time that meter reaches zero, your stock of digits increases. As you advance in levels, the Extend meter grows, making it harder and harder to stay in the game.
Making matches also gradually fills the Burst meter at the top, which sets off a kind of bonus round. During Burst Mode, matches can form with any combination of connected blocks, not just ones in a straight line. This is your chance to rack up a few extra blocks for your stock, if you can form a few large chains.
If you survive long enough, you may encounter some special blocks. Metal blocks keep the same digit no matter what, ice blocks must be thawed out by making a nearby match, and blocks with a circle of surrounding arrows will increment every turn.
Analysis: Let me address the timer issue, because I know that time limits are a real bummer for many of you. Yes, there is a strict time limit on each move, and logically, that will prevent you from playing the ideal strategy, thus stressing you out. In the timer's defense, you can't deny that it gets the blood pumping, and it also ensures that the game will end reasonably quickly. Without it, you could spend the rest of your life hunting for the best move every turn, and frankly, you've got other stuff you should be doing.
Zerosum is a great example of how to train a human brain to work in new ways. Initially, there seem to be limitless choices for every move, but you will begin to recognize patterns and opportunities. Out of necessity, you'll develop more and more sophisticated strategies; setting up future combos, sprinkling the board with zeros, planning three moves in advance. It's brain candy, once you get into it. Like defusing a bomb constructed by a six-year-old arch-villain.
This also demonstrates how Match-3 games like Bejeweled and Tetris Attack aren't a design dead end. They can be transformed from a casual and relaxing past-time to an intense and intellectual past-time, just by adding some adding. If you like this, you might also enjoy Chain Factor.