Choose a particular hazard, such as falling, being tripped, triggering traps, mind control (or another fairly specific power effect, such as Damage with the fire descriptor) or a particular skill with consequences for failure. If you fail a check against that hazard, you can make another immediately and use the better of the two results. You only get one second chance for any given check, and the GM decides if a particular hazard or skill is an appropriate focus for this advantage. You can take this advantage multiple times, each for a different hazard.
D20 Hero SRD Advantages Article- http://www.d20herosrd.com/5-advantages