When driving a Prius, you can view a bar-graph of your fuel consumption over the past 30 minutes. A "prius tetris" is when you go five consecutive minutes (or more) at 99.9 miles per gallon. This shows on your bar-graph as a long, straight bar, similar to the four-high bars in Tetris.
No wonder you don't need any gas! You're always getting a prius tetris!