Nothing. The word “slain” means “murdered,” and God doesn’t do that. Neither does He knock people over or cause them to lose control of themselves and laugh unstoppably, or bark, cluck, twitch, or gyrate. Those are not manifestations of the spirit according to the Bible.
The key principle here is that God NEVER usurps anyone’s free will. It is the Devil that infiltrates people’s minds via evil spirits and causes them to do such absurd things.
Here’s an article that thoroughly answers this question: “What is Slain in the Spirit?” Also for a very in-depth look at understanding the entire Holy Spirit field see: “The Gift of Holy Spirit: The Power To Be Like Christ.”
Here’s a video that magnifies the absurdity of this practice.