Warning: graphic content. This is what keeps things from falling out of the pocket.
Note: this feature took 50 years to develop, and attempting to duplicate this should not be undertaken lightly.

With gym shorts in proper position:

Layer of vinyl (for waterproofing) and fleece (for wicking of moisture) added
and stitched to form a pocket for ID/credit cards and such:

(Not that I expect to carry when overseas!)

Detail of back, showing layering: