I have blogged about being slain in the Holy Spirit. This had happened to me several times. Each time, a charismatic priest put his hand on my forehead to bless me, with or without some relic, and I just fell involuntarily to the floor. It's also called resting in the Spirit, for you experience deep peace in that state. When I went up the aisle to be blessed, I did not question if I was going to fall or not. By abandoning myself to the Holy Spirit, I did fall and experience the peace. The one time this did not happen was when I had some bothersome thoughts on my mind, preventing me from complete abandonment. So the lesson I learned was that if you wish to experience the wonders of God, you need to abandon yourself 100% to the Lord.