It's a gimmicked card used to create a break in the deck (for controlling where to cut). Earlier in the video, you can clearly see it (or another gimmicked card

It's a gimmicked card used to create a break in the deck (for controlling where to cut). Earlier in the video, you can clearly see it (or another gimmicked card) creating a break (gap) in the deck. The pop at 2:15-2:20 is a slight vibration (due to the flipping card) causing the card so slide or pop to become more visible.