Whenever something really bad happens to you, you may invoke this schtick. The bad thing will not happen, but something else will happen instead. Like, if you are about to fall of a cliff, you will instead drop your gun over the cliff.
You can recover Fortune points by inadvertedly causing setbacks. Your bungling causes troubles and loss of gear and temper to you and your pals, but when you need it, your own luck just never seems to run out. The GM has the final say on wheter your setback is suitably funny and annoying.
You can also cause setbacks to others by spending an Fortune point - as long as their Fortune is lower than yours.