At first the process is so fast, it appears to be folding the garment as if by magic. However I will slow down to reveal exactly how it is done.
First place the T-shirt face up on a flat surface. Then pinch the material halfway down and a couple of inches from the left-hand side between thumb and forefinger. With the other hand then trace a straight line up towards the shoulder to grab a spot to the left of the collar - directly above left hand.
Draws the top of the T-shirt down to reach the bottom left-hand corner. Then picking the whole item up flips it over so it is perfectly folded - collar facing upwards and sleeves tucked behind.
If it is easier, you can just watch the video - it has a slowmotion of the process.