The esophageal hiatus is an oval opening in (sources differ) the right crus of the diaphragm/left crus of the diaphragm, with fibres of the right crus looping around the hiatus to form a sling (upon inspiration, this sling would constrict the esophagus, forming a functional (not anatomical) sphincter that prevents gastric contents from refluxing up the esophagus when intra-abdominal pressure rises during inspiration).
The esophageal hiatus is an oval opening in the diaphragm's right/left crus, with fibers looping around it to form a sling that acts as a functional sphincter to prevent reflux during inspiration.