I think I know what it means, but I'm realizing that's not what is written. The intent is there, when halflings burn Luck, they get 2x the bonus to the roll. But that first sentence reads, twice the bonus of a Luck check. One can read it as being a halfling (using the roll-under Luck check) with a Luck of 12, needs to roll a 24 or lower... Or, they get their 2x their Luck mod on their Luck checks? That's just weird. It probably should read:... a halfling doubles the bonus of a Luck check. For every 1 point of Luck expended, a halfling gains a +2 to his roll.
First, a halfling doubles the bonus of Luck burned. For every 1 point of Luck expended, a halfling gains a +2 to his roll.