Welcome to my discussion of flagging in chess. I will be primarily talking about flagging in online chess. Since flagging occurs very rarely in over-the-board (OTB) chess, I will only mention it in passing. First of all, what is flagging? A 'flag' is literally claiming that your opponent has forfeited on time. 'Flag' can have a slightly different meaning, which is to play to win on time. I will be talking about the latter (although it is normally discussed as 'flagging' or in the past tense, 'I flagged him').
This only comes into play if your opponent is low on time. The typical flagging technique is just to play normal moves or possibly more careful/safe moves in the hope that your opponent runs out of time. Of course, this should only be done if there is no increment, as otherwise your opponent can acquire a good amount of time back very quickly and possibly exploit your imperfect play.
Flagging in Internet/Online Chess
Some online players believe that flagging is disrespectful and will even throw profanity at you if they become the victim of it. My personal view is that flagging is a perfectly legitimate technique to resort to if you cannot match your opponent in the position on the board. I am backed up by both Kasparov and the Internet Chess Club (ICC) help files. Kasparov, in the press conference after winning the last game on time against his old rival Karpov (in their match in Valencia, 2009), said that he would have preferred to win by playing the games out, but "the clock is also part of the game"... Click here to access the rest of "The Art of Flagging in Chess".