The description says the user uses the Demon's Power instead of his/her Stamina for determining actions... the Combat section says that everyone gets one action. Is Fast supposed to be used for determining *when* someone gets their action? And if so, how is that worked?

Roll dice equal to Fast initially. If this is higher than the PC's Stamina, separate out the Fast dice from Stamina dice somehow (roll them elsewhere, different color, whatever). The Fast dice only count towards "when you get to go", not towards performing the action.

Also, when using Fast to do "a little bit more" (per the rules), rolling the Fast dice to determine the success of this 'little bit more' is appropriate. [1]

Fast is like Armor. it's turned on once as an action at the start of a combat (or prior to the combat, given a demon with some brains and/or prep), and then continues to work without the need for further actions, and it all counts as one use. [2]

As for Fast, here's how it works.

1. The case of a demon who makes another person faster. Let's give the person Stamina 2 and the demon Power 6. Let's say the person is engaged in some sort of combat in which he and a bunch of other people are all fighting with tire irons, and the ability is currently on.

For that person, roll six dice, with four of them being a particular different color than the others. Set the order of that person's action according to all six dice. However, when that person's turn comes up, only use the two "real" dice (his Stamina) values for the effect; i.e., what the defending individual has to roll against.

2. The case of a demon who makes itself faster. Let's give the demon Stamina 5 and Power 6. Let's say it is engaged in some sort of combat in which it and a bunch of other people are all fighting with tire irons, and the ability is currently on.

For that demon, roll six dice, with one of them being a particular different color than the others. Set the order of that demon's action according to all six dice. However, when that demon's turn comes up, only use the five "real" dice (its Stamina) values for the effect, i.e., what the defending individual has to roll against.

Finally, note that all of the above counts no matter what action is being taken, and what score is being utilized.

[3]