I have tried this, requires arm motion more than wrist.
Hold your knife as normal (i.e. if you use the thumb stud), but don't touch the stud at all, thumb along the side of the handle to support grip (the grip must be clenched during the peak speed of the move)
Raise your arm about half between horizontal to vertical, in front of you, knife facing down (blade spine facing away from your palm), I call this +45 degree
Swing arm down, fast, with the peak flick at about horizontal to -45 degree.
Your arm stopped at -90 degree, blade will fall out and lock in place pointing down.
Another way to describe the movement is as if you are trying to cut/slash to -90 degree from +45 degree (if blade is already out), while at -20 degree there is an obstacle that you need to put force on.
I hope this is not so confusing.
