It looks like you are using an ad blocker. That's okay. Who doesn't? But without advertising revenue, we can't keep making this site awesome. Click the link below for instructions on disabling adblock.
Basically (if it's not clear enough in the video), it's a 360° sideward rotation (Barrel Roll style) while moving forward PLUS an extra 180° twisting rotation.
Honestly, I was attempting a Backflip 180°, so that's why I say it was "unexpected". Can I do it again? No. At least, not without practice.
I asked Reddit about it, and people mostly replied it was an underflip, but I must kindly disagree with this because to me, an underflip is mostly like a flair which is a 360° backward rotation with an overall 180° twisting rotation (maybe more like 90° at the start and 90° at the end in the case of an underflip).