Ok, i think the insertion method is right, he uses half a flap, the flap that would be on the inside once the second flap is pushed up.. which is cut just slightly with a slight bend inward, giving him a break at a specific card. I did notice when he pushes the card into the deck he does so at a slight tilt. I then think he pushes the opened flap that has adhesive up when he oushes the card in, with cellophane on it closed to do the reseal.... I think cellophane bends light so you cant see any edge cut. Most people will be looking for a slit in the cardboard. The stutter previously mentioned might be because of the irregularity in the cellophane.
His force can be multiple choices with a range ... It is possible to have only five or six cards marked, each with a corresponding deck in his pocket for that card. Say the five through ten. The marking system would be unkown to me, something with his glasses? Uv ink? No clue... Polarized ink spot? There wasnt much inspection of the initial cards, just shuffling.... With five cards, it would be hard not to have one of the five cards somewhere in the middle of the deck while having the spectator plop their finger on a card.
At least this is how i would do it.