To perform this trick, you need to prepare the banana by slicing it inside it's skin. Here's how you do it: At the top of the banana along the back curve, poke a hole with a toothpick and move the toothpick back and forth inside the peel so that it is slicing the banana. Repeat this every inch or so down the curve of the banana. When you poke the hole, be sure that the toothpick goes all the way through the banana, but not the other side of the skin.
Once you've prepared the banana, gather your audience. Hold the banana so that the curve where you poked the holes is against your hand. Show everyone that there are no slices or holes anywhere.
Then pretend to karate chop the banana. Make sure you don't hit the banana!
After you pretend to chop it, peel the banana and show how it's been magically cut into slices.