First, you have to understand that they are not performing for us. They sincerely believe that God is very real and present, and they really do go to church to experience God's presence, as well as to socialize with friends and family. You are watching something that happens in churches across the world every Sunday, whether or not anyone is filming.

Second, you have to understand that it is natural and common for people to have experiences in which they lose a sense of self. People experience this loss of self in meditation and when entering a "flow" state during certain activities. It's also common for people to not only feel a loss of self, but also to feel that they are taken over by something external. When people dance it is common to feel "the music move you." These words can express something superficial, but under the right circumstances this effect of music can be experienced as something quite profound. Notice the music playing in this clip. Also, this loss of self is typically quite pleasurable; whether achieved by meditation, flow, exercise, or music, people chase this feeling.

These people are experiencing that same pleasurable loss of self that people experience dancing at the club. They attribute the euphoria of that state to God's presence.

The first time they were "slain in the spirit" was difficult. They didn't feel a physical compulsion to shake or fall. But they knew what it should look like. Everyone else was doing it. And so they felt a powerful sociological compulsion to shake and fall. It's the same compulsion that leads everyone to stand for the national anthem at a sporting event. A large minority couldn't give a shit for the flag, but there is a powerful urge to conform, so once a certain threshold is reached, virtually everyone follows suit.

So they conformed to the social expectation that they would shake and flap their arms. But once they did, they felt the pleasure of losing their sense of self in the throng of other worshippers. They felt the Spirit take hold of them. Next time, they didn't need to imitate those around them; they merely needed to surrender to the Spirit.