Back in 2008, I spent a little time in Korea. One of the things I wanted to do that trip was eat live octopus, known as sannakji in Korean. It's not a particularly exotic dish there. In fact, just wait until the sun goes down and sannakji food carts will magically appear next to subway stations. One night, Mr. Bong, the owner of the hostel where I was staying, took me and another guest to his favorite.
Here's the owner of the stall preparing the octopus. First, it's taken out of the tank, its legs stretched out on a cutting board, and its body cut off. Then the legs are sliced into bite-size pieces.
The octopus has arrived. The tentacle pieces are still moving, much like a chicken with its head cut off. Grab a piece, dip it in sesame oil and then chili sauce. Pop it in your mouth but stat chewing right away. As you can see in the video, the tentacles are still working and the piece of octopus will try to stick to the inside of your mouth.
I recommend drinking soju while you do it.