You can ignore the effect of negative levels for a short time.
Benefit: Whenever you would gain a negative level, you can ignore the penalties and other ill effects associated with that negative level for a number of minutes equal to your Constitution bonus (if any). For example, if Tordek (Con 15) is struck by a wight, he gains one negative level. However, he can ignore the –1 penalty on attack rolls, saves, ability checks, and skill checks associated with that negative level for 2 minutes, since his Constitution bonus is +2. (If Tordek were a spellcaster, he would also avoid losing a spell slot for 2 minutes.)
You also gain a +4 bonus on Fortitude saves to remove negative levels.
Source: Libris Mortis, page 26.