Thanks for your question.
Being "slain in the spirit" is a phrase used by so-called charismatic Christians to describe their reaction to receiving the Holy Spirit. This is not a Biblical practice and it is mentioned nowhere in the Bible. It has much in common with pagan rituals and Caribbean and Brazilian Voodoo cults. It is just another aspect of the false un-scriptural practices of the Charismatic/Pentecostal movement. Here is some useful information from the website for you to consider: