The short version: a two battery Gold Mount hot swap for any Gold Mount camera, so you can change either battery mid shoot.
What you'd use it for
Bolt it onto any Gold Mount equipped camera and hang two batteries off it. Unlike stacking batteries, this is true hot swapping: pull either battery whenever you like and the camera keeps running.
Smart circuitry combines the readings from both batteries, so the camera's display and viewfinder show an accurate total.
In real world terms
Good to know
Works with
Any Gold Mount equipped camera.