Help with A star in Paris

Hey guys can anyone help explain or break down the opening slack from this trick? It starts froma houdini mount but I cant figure it out.
link is below


I had the same problem at first, when you do the slack you need to put your thumb in (like he does) and as you do the slack let the string slide off your thumb but pinch it with your throw hand index finger.

Hope this helps out a bit (I’m not good at describing parts of tricks).