There is no biblical support for the Pentecostal/Charismatic experience called "Slain in the Spirit".
For various reasons, most people who practice this experience fake it by willingly 'fainting', and/or are influenced by well-understood effects of body positioning/balance, anticipation, hypnosis, peer-pressure and pride. Out of ignorance of the scriptures (Hosea 4:6), the minority who are actually overcome by a spirit being, have yielded their members to Satan and are under the power of his devils. (Rom 6:16)
We are open to examining scriptural proof for this phenomenon, as is commonly practiced by Pentecostals/Charismatics. Pending our receipt of that, the reader may be interested in a personal testimony, as follows:
Learning the "Slain in the Spirit" Experience
1. Having been raised in Pentecostal churches, and actively participated in them for several decades each, (my wife since the early 1960's, and I since 1969), we are thoroughly acquainted with Charismatic/Pentecostal phenomena and doctrines. Our decision to reject all of their unscriptural teachings is not one that we made in haste. We can assure you that our family has done our homework, and - with equal assurance - can say that the baby is alive, but the bath water is gone.
2. I will briefly mention two of the dominant teachings from the movement that we have come to reject - slain-in-the-spirit phenomenon and speaking in tongues.
3. Having accepted Christ as my Saviour in 1970, and having attended some 48 Pentecostal churches since then (because of business travelling and moves, primarily), I believe that I have seen all there is to see, and have heard all there is to hear about core Pentecostal beliefs, thinking, doctrines, justifications, experiences, etc, etc. Moreover, my family and I were not simply attendees - we have been deeply involved, supportive, and evangelistic in our support - throughout these many years.
4. My first real exposure to slain-in-the-spirit phenomena occurred in 1974 at a Kathryn Khulman crusade in Peterborough, Ontario. I was awed by the fabulous claims and stories that were told about miraculous healings that had previously occurred and were occurring before our eyes (supposedly). I was fascinated by the demonstration of power as people fell to the floor when Kathryn laid hands on them. But, (to keep this short) I was disturbed by some things, most poignantly by the fact that a young girl - about my age - who was badly twisted from cerebral palsy (or from some similar condition), was pushed in her wheelchair out of the arena, in the same condition in which she had expectantly arrived - presumably for a healing - despite having had a great deal of faithful and sincere prayer extended on her behalf. I have seen this type of story repeated over, and over, and over again, throughout my experience. Despite literally thousands of opportunities to witness such things, I have witnessed no instantaneous organic healings at any time in my life - in anyone.
5. Two years later or so, Benny Hinn spoke at a prayer breakfast and I watched him pray for my mother on the other side of our table. The group of Full Gospel Businessmen who accompanied this budding superstar performed like a cadre of bodyguards around the president, raising great expectations and excitement, and exuding power and prestige simply by the manner in which they approached and moved through the crowd.
6. Mom was not slain-in-the-spirit, but she told me that - if Benny had tarried longer with his hand pushing her forehead back - she probably would have been. This was her somewhat embarrassed response to the fact that she hadn't been 'faithful enough, or yielded enough, or righteous enough' to have been blessed with this (supposedly) wonderful act of the Holy Spirit. (This was my first-hand view of the guilt implicitly - and often explicitly - placed on the seeker/ recipient).
7. I could relay over thirty years worth of stories to you, and the conclusion of them all is that we have (unknowingly) participated in a farce - deceived by people who knew better or who should have known better. Having studied God's word on the matter exhaustively now, we clearly denounce the teaching of slain-in-the-spirit. Nowhere is the practice found in the bible, and only on the enemies of God did an overwhelming force come to cause them to fall backwards. In every other case that is specified, men who came into the presence of God fell willingly forwards, on their face, (in humble worship) - or, ... they DIED.
Prove all things; hold fast that which is good (1 Thess 5:21) Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world. (I John 4:1)