What forces food back into the pharynx so that it can glide easily down esophagus?

The forces that move food back into the pharynx so that it can glide down the esophagus are called peristalsis and antiperistalsis.

Peristalsis is a series of involuntary muscle contractions that occur in the walls of the esophagus. These contractions start at the top of the esophagus and move downward, pushing food down into the stomach.

Antiperistalsis is a series of involuntary muscle contractions that occur in the walls of the pharynx. These contractions move in the opposite direction of peristalsis, moving food back into the pharynx from the esophagus.

Antiperistalsis is a relatively rare occurrence, but it can happen for a variety of reasons, such as when you burp or vomit.