Sacrifice To Power Spells Options

I'm wondering if anyone could explain to me a little more of the theory behind a sacrifice of some sort (not necessarily living, mind you) powering or empowering a working. I've encountered the concept a lot, but sacrifice as I understood it involved giving something to (the) god(s); can its energy be harnessed? Indeed, is there energy to harness?

Any and all thoughts welcome. (IMG:style_emoticons/default/smile.gif)

Frater U.D. says:
In the spirit model magic is seen as being effected by these entities who are usually invisible, at least to the average punter, and it is
the shaman's or magician's task to make them put his will into effect. This may be done by prayer, by barter, by cajoling or even -
vide medieval demon magic - by the application of magical force, threats and pressure.

Sacrifice or making offerings to a spirit or deity is something I can understand. It is barter or bribing the spirit. I have known people who prayed to saints and asked that if the desired outcome was successful (eg. their wife survives the operation) they would quit smoking or drinking or something of that sort. I suppose that this is an attempt to placate the saint.

I don't think that sacrifice works unless there is someone to sacrifice to. The exception is Frater U.D.'s Energy Model for magic. You may raise energy to power up the spell or ritual. For example, using sex magic to raise energy for a certain purpose. But the energy is used by the spell. It isn't sacrificed in the normal meaning of the term.

That's what occurs to me off the top of my head anyhow.