This article is from the GNU Chess and XBoard FAQ, by Tim Mann firstname.lastname@example.org with numerous contributions by others.
If you are playing with the ICS incremental clock, both you and your
opponent get a set amount of extra time after each move.
If your or your opponent has netlag, your opponent might appear to get
extra time, especially if your opponent is using timestamp or timeseal.
The ICS charges each player who is using timestamp or timeseal only for
the time between when the player received his opponent's move and the
time he sent his own move. Thus delays in network transmission do not
count against either player. But WinBoard counts down the display of
your opponent's clock on your screen under the assumption that there is
no netlag. When his move comes in, if there was netlag, the ICS may not
have really charged him for that much time, and WinBoard corrects the
clock to what the ICS says it should read.
If you are not using timestamp or timeseal, you may appear to lose time
off your clock at some point after you make your move. In this case, the
ICS charges you for the time between when it sent you your opponent's
move and the time it received your move. Thus delays in network
transmission count against you. WinBoard stops counting down the display
of your clock on your screen (and starts your opponent's) when you make
your move. When the ICS echoes your move back to you, it may have
charged you for more time than that, and WinBoard corrects the clocks to
what the ICS says they should read.
See "help lag" and "help timestamp" or "help timeseal" on your ICS for
more detailed information.