There isn't really much lore for how soul gems work for resurrection. The best we can do is extrapolate and theorise based on what info we have. We know that, as a Soul Shriven, the body is made of Chaotic Creatia. Most Soul Shriven are imperfect copies of their living bodies, because they have lost the focusing principle of their Anuic souls, but as the Vestige is a "paragon" Soul Shriven, their Anuic valence is so high, their body is reformed perfectly and instantly upon contact with Chaotic Creatia. Usually on death, a Soul Shriven would return to the Waters of Oblivion like a Daedra, and reform another imperfect body.

Here's where the theorising starts - at the end of the Wailing Prison, we absorb a Skyshard (Aetherial fragment) to be reattuned with Mundus. This likely severs the connection with the Waters of Oblivion, meaning the Vestige will not return there on death. Instead, when we die, the body remains in-situ until either:

• a soul gem is used. Since a soul is an Anuic focus, it is likely that soul kick-starts the Vestige and the Chaotic Creatia reforms the body instantly.
• you release to a Wayshrine. A Wayshrine is a gateway to Aetherius, so once again, it is likely that connection kick-starts the Chaotic Creatia to reform the body.

nine9six wrote: »

Why am I able to Soul Trap something and then use Repentance?

Repentance consecrates the souls of the fallen. If you've stolen the soul with Soul Trap, Repentance will still consecrate the body, thus meaning necromancers will not be able to raise it (as necromancers cannot raise the consecrated dead).