Compel : This technique allows the psyker to force others to briefly act against their will. The psyker makes an Opposed Willpower Test against the target. If the psyker succeeds, the target must follow his commands. The commands must be simple and achievable in one round. Some examples include “Flee,” “Fall,” “Attack the closest target,” and so forth. If the command is a potentially suicidal act, the target gets a +20 to his Opposed Willpower Test.

My question is simple: When? As in, when does the compelled action take place? The power gives no hint as to when the target must follow the copulsion. Does it happen immediately upon manifesting the power? Or does it happen only when it becomes the target's turn in the initiative, which would be the most logical explanation. We're presented with these two options.

A: If the compulsion occurs immediately upon manifesting the power, you run into problems such as an astropath compelling a teammate to attack a target. Then when it becomes the teammate's turn, he attacks again, this time of his own volition, essentially giving the team member two attacks at the sacrifice of a half action from the psyker. A character is allowed one attack per turn , and since the teammate would be attacking once on the astropath's turn and once on his own turn, this would technically be allowed.

B: And when we think about the compulsion occuring during the enemy's turn, then that adds another facet of usefulness to the power. That means that not only can you make someone else do whatever you want, you also strip them of their next turn by making them act on their turn, not yours (in the case of the 1st argument).

How have all of you handled this? Or am I just grossly missing something?

A: You are correct. If you look in the Errata/FAQ (check it out online on the website here under Rogue Trader support) it says:

This power seems mostly used ouside of combat, but even when used in combat it doesn't change the speed or efficiency of the Compelled character. He does not suddenly get to act twice in a round, make twice the number of attacks or twice the number of Move actions.

He is simply no longer in charge of his faculites, no longer calling the shots.

I rule B. He follows his usual Initiative order, uses all his regular stats and abilities, but may not choose freely from his avalable Actions. If he was compelled to Attack target X, then that's what he'd do, using his most effective option (melee/ranged, aimed/auto, whatever he would usually do if voluntarily attacking the same target).

And thats all there is to it. Don't overcomplicate things

That's what I figured. But I do have a question: How do you figure that compel is a mostly out of combat power? Not only is every example they list in the description an in-combat use, the power is non sustainable. Meaning you use it, and you're done. It lasts for a round. Which pretty much exclusively means in-combat power, since rounds are only used in combat.

Well, it certainly has tons of out-of-combat uses, but because it's so short, typically only in action scenes that will be measured in rounds anyway. Chases are fun when you can order the guys chasing you to fall down…. sentries can be made to close their eyes or take a walk the wrong direction… street urchins to throw rocks at rival Rogue Traders… bonus points if you get their friends throwing rocks without Compelling them to, hehe.

In-combat, I rule that having someone turn traitor for a round on the guy next to them armed with the multi-melta or the twin autopistols is a "suicidal action", and so it's easier to have someone do more creative things than to make them an ally for a turn. I think I had Lady Ash in the starter mission Compel the group's best shooter to throw his weapon at her bodyguards instead of shooting it…. he had quick draw, though, and threw his sword instead so he could keep his gun