I have antilag set on the server to level the playing field so the people with a lower ping don't have an unfair advantage over less fortunate players with higher pings.
Its a latency-based system which uses algorithms to adjust hit registrations according to a mean average between 2 players whose latency (ping) is plus/minus 50 difference between them. It will delay the rigistration for faster connections, and speed it up for the slower one.
In practice, in speeding up the registration, this sometimes appears like you've been hit before the player has even taken a shot. But that is only an illusion created by the algorithm used.
Between players whose latency (ping) is not greater than a certain mean average between them, no noticable delay or speeding up is made.
In other words, it gives slower ping players a fighting chance against players with a much faster ping. But between players whose ping is roughly the same, no such adjustments are made.