Question

how food moves in the opposite direction during vomiting

Answer 1

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

Answer 2

When a person vomits contents of their stomach are pushed back upwards by reversed movement of the muscles of the pipe. At the same time we can observe extremely abundant production of saliva which is supposed to protect our teeth from digestive acids.

Hope it helps you.