I learnt this while training with some Grand Dragons. I think I learnt it because of either a confused character (can confused ones use items in IX, it's been a while since I played it :S) or cos I was looking for an item and selected and confirmed a tent by accident.

I use it every now and then on enemies since I have a ton of money in the later stages so can afford to waste them, but most enemies by then are immune to that snake