How exactly does this psychic abilities? Astropath penetrates into the mind of the victim and what happens next? He gets full access to the victim's memory and pumps all the information in your mind, getting a split personality? Or he can interrogate the victim only time that he sustain this psychic power, extracting answers directly from the mind of victim and as soon expire ability astropath loses access to the knowledge of the victim?

For example, consider a four level (subconscious) .
If I do a fourth level mind-probe the mind of another Rogue Trader, I know all that knows the trader, will know his Secret Cant (RogueTrader), cipher from its treasure vault, its trade routes and all rest for five rounds?

P.S. Sorry for my english, I am still working on improving it.

I really hate this power for basically these reasons. It's quite vague as to how it works, which means every Astropath that tries to use it will interpret it to be the most powerful ability ever.

My ruling for how it works is this: When you use it, you gain access to that person's memories, and that's it. You theoretically know everything that they know, but you have to ask specific questions to the GM (i.e. me) to see if that person knows something. You don't just instantly start going through their life one day at a time to get everything they know, because then you will be sitting there for days at a time going through what they know. You can't use it to simulate their actions because all you get is their memories, not their private reflections on them, nor their understanding nor interpretation.

For something that the person would have to be explicitly taught, then I have allowed my Astropath to spend a lot of XP to get an Elite Advance corresponding to that exact skill - Ciphers being an excellent example - but otherwise the memories fade when the Mind Probe ends for exactly the reason you've outlined.

I have a different issue with Mind Probe, related to the way the affected person behaves during and after the Probe has finished: there is a contradiction in the manual between the description of the process (it's a brute force process that requires sustained effort from the psyker) and the option to cast it in such a way that the target will not know he has been probed (at -20 Opp WP).

I'd really like to be able to probe people without them knowing, but it's kinda hard to understand how to apply the "not knowing he's being probed" rule, as long as the process itself is described as a struggle. The -20 could mean I'm doing extra efforts to remain "unseen" in the mind of the target, but it's a bit of a stretch (it could maybe work just for the first couple of steps in the Mind Probe chart).

I'd really like to be able to probe people without them knowing, but it's kinda hard to understand how to apply the "not knowing he's being probed" rule, as long as the process itself is described as a struggle. The -20 could mean I'm doing extra efforts to remain "unseen" in the mind of the target, but it's a bit of a stretch (it could maybe work just for the first couple of steps in the Mind Probe chart).

I think the fluff text is for the "brute force" approach, and doesn't apply to the subtle approach. As an analogy, one way is like storming the gates (lots of fighting), the other is sneaking in through the sewers (no fighting at all).

Fear the psyker with Mind Probe and Total Recall!