This is a "Tamsinism" for the "extra" liberties that a large eye of 4 liberties effectively provide in a semeai.
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:
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. In effect, this four-space eye gives five virtual liberties.
The number of virtual liberties increases sharply with each extra space in a large eye?, and this can be remembered using the SemeaiMnemonic, which is 3=3, 4=5, 5=8, 6=12.
Please don't point out the above example is already alive to begin with. I used it only to show the principles involved.