As far as I know, the BG essentially ties the two batteries together in parallel. I'm sure there's more to that since they're LiIon batteries, but the basic idea seems to be right. If you insert batteries of mismatched charge, they may equalize, but it's saying that you shouldn't lose any relevant charge in the process.


The ON switch on the side of the grip merely tells the grip whether to ignore or listen to the shutter, wheel, and button inputs on-grip. Some people buy the grip ONLY to carry two batteries (or use the 6xAA tray) and don't want the grip's controls to be active, and that's why the switch exists. I know that I've accidentally shot because the camera bumped into me and "I" pushed the shutter button. I also seem to hit the FEL button on my 1D3 often, triggering a test flash at the most random of times.