Talk:Emperor Palpatine/@comment-37.9.184.181-20180218200625/@comment-27029853-20180218203231

That's from the FAQ.
 * When attacking or defending, Emperor Palpatine is used during the “Roll Attack Dice” or “Roll Defense Dice” steps respectively.

So you choose to call Palp before rolling, roll dice, Palp immediately changes one of the results, and then the attacker then modifies the defense dice.