Buttering?

Hello, I watched a video of a guy buttering, and strangely i’ve never seen it before, and looks like great fun! I am an intermediate rider i’d say (been riding for 4 years) so i can do some basic tricks (im not really into tricks, just a few small jumps and jibbing) but this buttering thing caught my attention and i’d like to learn it. Is there any good video that explains how to do it