OOC:
I'm not sure about the page number or anything (don't have the book), but I found it in a basic explanation of the rules in an online review. You can (supposedly) spend luck points to improve a roll, but it drops your luck score, and makes it harder to succeed when you have a luck roll.
Example: Jon needs a 35 to succeed at shooting a monster with his revolver. I roll a 40 instead...Currently, Jon has a Luck score of 25. I spend five of those Luck points to bring my final luck to 20, and my roll of 40 becomes a roll of 35...success!
From what I'm reading, that was in the playtest packet people got from the KS, but it's not in the official rules anywhere...I won't know for certain until I get my hands on the book and look for myself. :/