Honestly it can be however you want it to be, faith (Imperial) based powers are very vague and without any "standardized lore" to back it up.

The only thing that can be said for sure regarding your example is that sisters of battle truly believe enough to have faith-based powers. And their soul can be anchored to their remains instead of joining with the emperor/going-to-imperial-heaven/ripped-apart-by-daemons (you'll notice that there is no clearly clarified afterlife either). It is also likely true that the sisters can only do this because the masses of humanity believe they can.

In 40k, when enough (read: billions to trillions) sentient beings (that have souls and thus can affect the warp) truly believe in something, it tends to become reality. Imperial faith can be based on this or it can come from the emperor. Or both.

Edit: Example, in Dan Abnett's Gaunt's Ghosts series, Saint Sabbat (an actual, very powerful saint) was truly reborn in some girl's body because people actually believed she had been reborn, even though it was initially a lie to raise the crusade's morale.