I'm curious to the exact specifics to this since it's not mentioned within the book.

I'm assuming that it's only a +1 to your Psy Rating so that you can roll more to manifest, but I want to ask to confirm that it doesn't act like the other Psy-Ups from Base DH which gives 1/2 WP bonus in Minor/Major Powers (Or a new Discipline).

Currently its open to interpretation by GM and players.

There is the argument that on a Primaris Psykers get far to much anyway and that the advancement does not work like the normal Psy Rating Talent and instead just boosts the rating by 1.

The counter to that is that the book uses psy rating +1 to represent the fact that ascended psykers can have different starting psy ratings and a generalised talent was put in place to represent the fact that psykers can have vastly different psy ratings when entering an ascended career and so gives out extra powers as normal, in support of this is the Inquisitors and Interregators psy rating +1 advancement, given that without the normal talent rules they are absolutely hosed when it comes to getting powers.

I personally am inclined towards the latter argument although that is persnal perspective rather than anything concrete.

Banjulhu said:

For me its very clear that this is the intention of the description.

My view is that it is purely a +1 to Psy Rating and no powers are gained through it.

If you come to the Psyker party late that's your problem. True psykers, those training and developing their whole life, will be vastly more flexible and capable.

I also support the view that a Psy Rating +1 works exactly as a Psy Rating 3, 4 or whatever. Alone the two different Psykerpath have a different Psyratings in the End, and it would be strange that a vastly superior Psyker dont get new Powers.