In Tetris Battle on Facebook, by some condition, the game tells you that you've "overwhelmed your opponent", and it grants you two stars rather than one.
What is that condition?
Your ranking at the end of the game determines how many stars you earn. Your ranking is determined by the following categories:
Number of KO's: If you force an opponent to the top of his screen by dropping blocks, you will earn a "KO." While this is usually the determining factor in terms of wins, when playing with multiple players, if several players KO someone, other categories are considered.
Number of lines sent: If you send more lines than your opponent(s).
Tertromino height: How high your Tetromino reach in your matrix. The lower the stack, the more points you will earn.
If you beat your opponents in all three of the above categories, you will earn three stars ("overpowering the opponent").