Man, this is really the $100,000,000 question....
My answer to that really depends greatly on how far down the rabbit hole you want to go. In any case none of the answers are really 100% satisfactory to me and raise more questions than they answer, hence the rabbit hole.
There are two major schools of thought here:
1) We have been given ultimate spiritual dominion over this realm - (this "realm" meaning reality as we understand it) There are a few bible verses that I can't think of off the top of my head, but they basically state that Christ gave us dominion over the spirit in this world and while they are here interacting with us they are completely subject to our wills. If we order them out they have no choice to comply.
While I suspect there may be a large amount of truth to this, I also suspect that it is not necessarily limited to christians. There are many religions and spiritual practices around the world that involve binding, manipulating and coercing spirits by prayer or ritual means. No matter who gave us the authority or whether the authority is just some sort of natural law we don't understand yet, these spiritual practices have been using this principle for thousands of years with relatively good success. This is whole concept is the whole logic behind why the catholic exorcism ritual works.
2) It really has nothing to do with being forceful. It has more to do with being able to motivate / manipulate your own spirit (put it into motion) in order to impose your will over the spirit's and thereby push them out. The ACT of being forceful or performing some sort of ritual gives you the focus necessary and bolsters your belief to the point that you are able to push the spirit out. The ritual itself is meaningless without your focus and spirit / will behind it, but without the (or any) ritual to GIVE you focus or belief it wouldn't work.
A good illustration that backs up this theory would be that we have on occasion seen a client completely and utterly fail the process of us coaching them through a house cleansing to the point that the spirits laugh at them. The spirits are often annoyed / weakened by some of the materials we use in the rituals, but are otherwise unharmed and still present. In these cases we've found that if we question the client and do some digging into how they really feel about what's going on they often have some serious doubts about what they're doing and what's going on. Ultimately they have yet to fully buy into what's going on which has in turn caused them to fail to utilize their faith / motivate their spirit to get behind the ritual. Their words are hollow and have no real meaning.
In those same situations if myself or one of our team who is a "believer" were to resume the ritual the spirit is quickly and effectively dealt with / pushed out. However, this effect on a client's home is usually only temporary. At best it lasts only a few days and sometimes only a few hours before the spirit returns. In these cases we refer to this as the client failing to declare their authority / dominion over their space / home. There is something powerful about motivating your spirit / faith and declaring a space as "yours".
As a further complication, people have the ability to channel light / energy and project it with the proper coaching and / or training. People who are spiritually awake / aware are especially adept at this. One of the first things we teach a new group member to do is to connect to that light, channel it through them and form it either into a protective shell OR use it as an offensive "weapon" to push a hostile spirit away. There are two keys we've found to this that are the major gauges of success. 1) the person seems to have to have faith in some kind of higher power or at least believe that there IS some sacred place for the light to be channeled from. 2) the person has to focus and clearly visualize. Visualization is especially key. The more you can focus / visualize the more effective it is. Tony is especially adept at this technique and I've met a few clients who when worked up can almost blind a medium standing in the room next to them when they channel the light. (yes.... I'm fully aware of hose hokey and full of crap this all sounds to some people, I struggle with that myself at times, but if it's stupid and it works.... it's not stupid, and trust me... it works. I've seen it work time and time again.)
Over the years it's become clear that for WHATEVER reason, spirits are subject to our wills. Some spirits are easier to deal with, others are more difficult, but I have yet to encounter one that is immune and I've run into some pretty nasty spooks over the years. Whether you believe it's because God has given us this authority / ability, because it's a natural law because this realm "belongs" to us or just because you are declaring space as yours and infusing it with your energy / will, the reason is almost irrelevant to the fact that the process works. I have spent a LOT of time myself wondering why and how the process works too.
Merddyn - And I took the one less traveled by And that has made all the difference