How do they stop the clock?

In at least half the games that I play on this site, there are times when my opponents clock appears to have stopped.  It is his move, his connection indicator is flashing, and his clock is not running.

Is this the player using vacation time, or is there some other way to pause the clock? If this is something that the player causes to happen, I consider it cheating. A game should be played in the time allowed, not in that time plus time gained by artificially stopping the clock, and such a technique would be impossible in otb chess.

If this is players cheating, I am not surprised as I have grown accustomed to bad sportsmanship on the site (e.g. many players let their clock run out rather than resign in a completely hopeless situation).

Oh well, I suppose cheating is alright if you get away with it . . .