Power from Pain is applied per battle round, not per player turn. So all units with this ability would have a 6+++ from the start, even if your opponent gets the first turn.
As it's stated in the codex:
- Quote :
- Units with this ability gain a bonus depending upon which battle round it is, as shown in the table...
In terms of the Black Heart, in battle round 1, every Kabal unit with PFP would be treated as though it was battle round 2 - so those units would have a 6+++ and re-roll charge and advance rolls. Whereas models in this obsession without PFP - vehicles and the CotA units - would simply gain the 6+++.
Apologies if I've missed or misinterpreted anything.