I recently got a baby Firefly, and the first time I picked him up, he was fine in my hand for a little while, then he sniffed my finger, slowly opened his mouth, bit down, and constricted. I hadn't touched a mouse or rat in several days, so I have no idea what made him do this. I filled a large bowl with lukewarm water and dunked my hand in there with him constricting it. he instantly let go, and has been fine ever since.
Just my experience, he was very small, I have no idea if this would work on a larger snake.