Being “Slain in the Spirit” is not biblical. It is not found in the bible. Falling backwards in the bible is a sign of God’s judgment. Beware for the bible says that in the last days false prophets will arise showing sign’s and wonders. The bible says that Satan can appear as an angel of light to deceive the people.

Genesis 49:16-18

¹⁶"Dan shall judge his people
        As one of the tribes of Israel.
        ¹⁷Dan shall be a serpent by the way,
        A viper by the path,
        That bites the horse's heels
        So that its rider shall fall backward.
        ¹⁸I have waited for your salvation, O LORD!

1 Samuel 4:17-18

¹⁷So the messenger answered and said, "Israel has fled before the Philistines, and there has been a great slaughter among the people. Also your two sons, Hophni and Phinehas, are dead; and the ark of God has been captured."
¹⁸Then it happened, when he made mention of the ark of God, that Eli fell off the seat backward by the side of the gate; and his neck was broken and he died, for the man was old and heavy. And he had judged Israel forty years.


John 18:1-6
 
¹ When Jesus had spoken these words, He went out with His disciples over the Brook Kidron, where there was a garden, which He and His disciples entered. ²And Judas, who betrayed Him, also knew the place; for Jesus often met there with His disciples. ³Then Judas, having received a detachment of troops, and officers from the chief priests and Pharisees, came there with lanterns, torches, and weapons. ⁴Jesus therefore, knowing all things that would come upon Him, went forward and said to them, "Whom are you seeking?"
⁵They answered Him, "Jesus of Nazareth."
Jesus said to them, "I am He." And Judas, who betrayed Him, also stood with them. ⁶Now when He said to them, "I am He," they drew back and fell to the ground.