Question

how does food move in opposite direction during vomiting

Answer 1

It usually occurs as a precursor to vomiting. Local irritation of the stomach, such as bacteria or foodpoisoning, activates the emetic center of the brain which in turn signals an imminent vomiting reflex. Food then moves in the opposite direction, often from the duodenum into the stomach

Answer 2

because of the pressure of air that is trapped in between the food, gives pressure and eject the food out.