While cleaning out my mailbox, I found a 2006 e-mail to Lupa on the distinction between possession and invocation. (This was written during the period when Raven Kaldera and I were soliciting information for Drawing Down the Spirits). I thought it might prove useful for those interested in the topic: you may also want to read Raven's essay on possession, "The Path of the Horse."
In an invocation, the magician calls upon and then draws into hirself the energy of a particular spirit being. This energy can have some pretty powerful psychoactive and spiritual effects. It can result in short-term and even long-term changes in behavior, have positive and negative consequences for your health, be used to effect real-world real-time changes, etc. But the deity doesn't take control of the situation. The user's psyche may be altered by the rush of spirit-energy but it's not kicked to the curb. S/he remains in control of hir body and mind.
Invocation is a necessary precursor to any possession ritual. (I might say it's an integral part of any successful magical operation). Any invocation worthy of the name draws the attention of the spirit world. Many of the techniques used to raise the energy -- meditation, drumming, dancing and prayer -- can also be used to encourage a possession. But in most instances invocation does not culminate in possession but in a general feeling of post-orgasmic bliss and good will. Please note that I'm not minimizing invocation's importance. Most conventional religions work almost exclusively with invocation. By and large invocation is safe, and if done well can have powerful beneficial effects. Invocation can heal disease, improve luck and weave powerful magic. But in and of itself invocation is not trance possession.
Invocation's cousin evocation works in a similar fashion, only in evocation the energy is directed to a particular area and constrained therein. Indeed, when using evocation, the magician takes pains to make sure that the entity's energy does NOT mix with hir own. If possession has a polar opposite, evocation is it. But that doesn't mean we can't learn from it. Like possession, evocation has a reputation of being extremely powerful and exceptionally dangerous. Many of the safeguards used in Goetic evocation can be applied to any possession ritual to ensure things go as smoothly as possible.
A few traditions use what has come to be known as "aspecting" or "shadowing." They become a vessel for the Divine or the spirit and move with the spirit. The Divine moves through them, but they never completely "step out of the way" and let the Divine take over. There's always an option to stop and regain complete control, whether or not the aspecting/shadowing magician wishes or chooses to use it.
Among most of the Religions of the Book, even attempting this would be a major sin. Stating that you were speaking as Allah, YHVH, Jesus, etc. would be the height of hubris. Consider the revealing of the Q'uran: Muhammed claimed that he was receiving Allah's Word through Jiv'reel or Gabriel and then reciting it, not that Allah was speaking directly through him. (Eliade might describe the Prophet's recitations as so many "spirit journeys," a technique he contrasted favorably with possession). But it appears to have been quite common in many pre-Christian cultures. Many of the public Greek and Egyptian rituals appear to have involved aspecting, for example: while I don't know enough to say with any kind of authority, I suspect the same is also true of many of the Mesoamerican religions as well.
Done right, aspecting may be all you need to speak with Deity and have a completely effective ritual. A good priest/ess who is able to subsume hir ego and who is able to listen to Deity can function as a channel without losing complete control. As Raven & Co. put it, they get to remain in the driver's seat while the Deity gives directions. By treating aspecting as a sacred encounter, not as playacting or rote recitation, a group can incorporate some lighter aspects of possession into their Work with relative safety and with great benefit.
Now we get into one of the great Possession Debates: where does "aspecting" end and "channeling" begin. At what point does it cease to be the Priest/ess speaking for Deity and Deity speaking through Priest/ess? When is it no longer you but the spirit moving the body and speaking the words? This is a hotly-contested issue, and one for which there are no easy answers. I've seen many borderline cases, and I've seen situations which started out as aspecting and became full-on possessions.
(I also would note that full-on possessions generally end pretty definitively, with little or no shading into aspecting or invocation-level energy, at least in my experience. Once the spirit gives up possession S/He usually goes away, with a palpable decrease in energy).
A common rule of thumb among Afro-Caribbean traditions is that a possession is only valid if the horse loses all consciousness. In my personal experience, I've found this misleading but not entirely untrue. When I've been hit by a lwa I've generally felt like I was watching the possession from a distance. I was somewhat aware of what was going on, although there were greater or lesser memory gaps afterward, but I certainly wasn't in control of things. I believe Raven & Co. call this "back seat, watching behind the safety glass" level of possession.
However, a couple of times I have lost consciousness altogether for the duration of the ride. These typically happen with Danto, who's known to be a rather hot and fierce spirit. These full-on "locked in the trunk" possessions are pretty intense but don't typically last more than a few minutes. (Although honestly I wonder if anyone could physically sustain any level of Danto possession for much longer without shorting things out). I will also note that I've found most possessions involve some level of memory loss, typically around messages the lwa gave to someone else. I've also had fugues where I attributed things to other people: thinking "somebody just got Ogou" when I heard myself shrieking, for example.
To add yet another layer of complexity, I've heard people who work with totem or animal spirits describe a variant form of possession. Instead of letting the animal spirit get inside their heads and bodies, they get their heads and bodies inside the spirit. They wear the spirit like a hat; while they give it some control over their bodies, their intelligence and ego remains more or less intact. Based on some cursory research, I suspect this was the case for many of the pre-Christian warriors and shamans who inspired Europe's "werewolf" legends.