My flattening method is to hold the shoulders and use a wrist-snap motion to snap it out, then flip it out to one side and down into the position for folding. This works really well, but has the downsides that, if done too vigorously, ① it can be really loud, akin to cracking a whip; and ② it creates a breeze that can knock over light objects, rustle papers, etc. — do not point T-shirt at other people, cluttered desks, etc.
My flattening method is to hold the shoulders and use a wrist-snap motion to snap it out, then flip it out to one side and down into the position for folding. This works really well, but has the downsides that, if done too vigorously, ① it can be really loud, akin to cracking a whip; and ② it creates a breeze that can knock over light objects, rustle papers, etc. — do not point T-shirt at other people, cluttered desks, etc.