Various tiny fish are dumped into a small bottle of overproof white rum, which causes them to jump like mad. The bottle is then whipped up with a vigorous spinning maneuver, and then poured down the gullet.
For every fish you drop, a bystander will traditionally give you a spank on the arse.
If you can keep it all down, the house will buy everyone a free round; if it comes back up, the next round is on you.