In cinematography, flashing is a method of contrast enhancement that takes advantage of the natural physical properties of film stock to bring out detail in darker areas of the print.
The effect is produced by adding a very small and even level of exposure to the entire image. Since exposure levels increase logarithmically, this tiny level of additional exposure has no practical effect on an image's mid-tones or highlights, while substantially boosting the shadows of the image, thus allowing darker areas of the image not normally exposed to become marginally visible.
It can be applied to the film stock before, during, or after principal photography of the film, although always before the film's processing. When applied before or after shooting, this is generally done by a motion picture film lab. The level of flashing needs to be tested ahead of time and subsequently moderated appropriately against the light levels of the scene, or else it risks having minimal impact if too low or making the shadows "milky" when too high.
On-set flashing solutions include Panavision's Panaflasher, which is mounted in between the camera body and the camera magazine throat, and Arri's Varicon, which functions as an illuminated filter and can be viewed directly through the viewfinder for manual setting of the flash level.
Flashing is usually described as a percentage of exposure increase to the film's base fog level. While the flash itself is often a neutral color temperature, the flash exposure could be any color: the color of the flash will be imbued disproportionately into the shadows of the image.