The pack cuts in two, both halves bow under the thumbs, and the cards spring off one at a time. The cards bend while it happens: for the length of the shuffle each one is a mesh rather than a sprite, so it can be bowed and tipped rather than only moved.
The order is decided before any of this runs. What you are watching is a riffle worked backwards from the deck it has to produce, so the card on top afterwards is the card the shuffle put there.