I know that understanding gastric motility is very difficult. Then, some years ago I found that Prof. Dr. Hansjörg Ehrlein (University of Hohenheim, Germany) had made an actual live video recording an incredible videofluoroscopy of gastrointestinal motility. He also made some animations explaining how is the mechanism. Luckily they are on YouTube so below I will direct you to some videos. You can also find his explanations

Gastric Motor Function

Gastric Motor Regulation