Effect can be as simple as increasing "brightness", or as complex as to include "layering" or other hi-tech photoshopping techniques.
Best results are achieved in photos with subject wearing sheer or single-layered clothing, or photos that are taken with flash photography.
Desired result is enhance photo to provide a clearer view of subject's "private parts" through their clothing. Breasts are usually the primary target of sexray photos.
"Fans of the sexy starlet manipulated her photos to create excellent "sexrays"."
"The red carpet flash photos of the Oscar winner in the form-fitting dress made it easy to photoshop "sexrays" of the photos with dazzling results".