gweinel
Explorer
You can also spend one luck point when an attack
roll is made against you. Roll a d20, and then choose
whether the attack uses the attacker’s roll or yours.
The aforementioned quote means that you have to see the dm's roll first or you have to roll and then the dm will tell you what he/she rolled?
If you can see the roll beforehand then it would pretty much beneficial almost every time you use the feat. Otherwise there is a strong possibility to use (and waste) the feat on the low rolls of the game master.