For instance, if you have a group with a four-space eye, that actually yields, in effect, more than four liberties. This is because the opponent has to make approach moves to put the group into atari (See Almost Fill.):
In the above position, White must approach three times to give atari, to which Black can reply by capturing, leaving two further liberties for White to fill in order to give atari again. this four-space eye gives Black five virtual liberties, the same number of liberties as White. Therefore, Black can win the race to capture if he plays first.