Board Thread:Warhammer 40k General Discussion/@comment-9558536-20130517084538/@comment-4264245-20130520084454

Gods in 40k appear in the Warp due to common beliefs. If a lot of people think a Daemon is a god and worship it, it will become more and more powerful. Same way, a lot of people think the Emperor is a god, but not as many as Chaos worshippers. Therefore there is a Warp power associated with the concept of the God-Emperor (from which the actual Emperor might draw energy).

Apparently, when someone is really devoted to a warp power, whatever its origins, he/she gets psychically bonded to it, and in a really stressful situation (a fight, a triumph, a very grievous wound) receives a surge of warp energy. With Chaos, since it's a very strong faith, you get such a big dose you can mutate or even become a Daemon. With Gork and Mork, you get berserk xD With the God-Emperor, this gives you more strength for a while, makes you able to banish Daemons through willpower or gets you back to life. In most cases, however, Warp energy hurts as much as it heals, and after the fight you usually drop in a coma (look at Keeler, for example) or worse ("the Emprah allowed him to win this fight before calling him to his side, all hail the Emprah!")