In the stomach, chyme is churned by peristalsis into the antrum, which contracts, reverses the flow, and allows more churning. because the pylorus is contracted during antral contraction, gastric content empties into the duodenum:

Answer: between contractions.

Gastric contents can empty into the duodenum between antral contractions that block the pylorus. The pH, fatty acid content, and osmolarity of the chyme stimulate cholecystokinin and gastric emptying. Constriction of the pyloric sphincter prevents the backflow of gastric contents and allows them to flow into the duodenum at a rate commensurate with the ability of the duodenum to accept them.