Snake handling is a late 19th/20th century phenomenon that has its roots in Pentecostalism but is not a part of mainline Pentecostalism. Services include speaking in tongues and other ecstatic worship experiences, but eventually the faithful break out poisonous snakes and pass them back and forth while drinking strychnine. The churches that practice this are generally extremely rural, but every few years one of the members makes the news for dying after being bitten.

The basis of these practice comes from Mark 16:17-18:

And these signs shall follow them that believe: In my name shall they cast out devils; they shall speak with new tongues. They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.

And also Luke 10:19:

Behold, I give unto you power to tread on serpents and scorpions, and over all the power of the enemy: and nothing shall by any means hurt you.

I understand how a certain interpretive hermeneutic could lead to the practitioners to certain infrences, but how they drew the conclusion that these verses meant that snake handling should be and/or must be incorporated into worship is lost to me.

The practice is technically illegal in Alabama, Kentucky and Tennessee, which have laws on the books that disallow the display of poisonous snakes in a way that could bring harm to others. The Kentucky law specifically mentions religious services.