This is what I have learned from attempting to operate the electric grip on the Horseman ER-1 camera.
The camera+grip and 4AA batteries was set up with an ER lens on lens board with its built-in solenoid and an
ER-type 6x9 back with the electrical contacts. You must load film in the back to test operation. I loaded
the film but did not fully advance it before turning on the grip.
The green “OK” light must be on to operate the solenoid on the lens panel. Given this, when the switch
is depressed, there is a light buzzing sensation and it attempts to operate the solenoid on the lens board
with a momentary single stroke. If the shutter is not cocked, it does not complete the attempt and the
light stays green. But if the shutter is ready, it depresses and releases the shutter. (N.B. Because the
operation is temporary, it does not work with bulb setting.). Then the light extinguishes and the red
light “WF” goes on. The release button has a lock and accepts an ordinary cable release.
WF, or wind film, is connected to the film back status. You must release the silver lock on the back and
complete film advance before “WF” dims and you then get the green OK again. So, (maybe) don’t load
the film and advance to “1” before mounting the back. To my surprise, you can remove the back, wind
the film off camera, and return the back and still get an OK for the next frame. This means, I think, that
you will be able to use the ground glass back to check focus/composition without problems. With no
more film to waste, I couldn’t check the functionality more thoroughly.
The red “DS” light, or dark slide, only seems to operate when the back is mounted, but is a rather simple
check light for removing the dark slide, dimming off in my hands only when the slide is completely
removed. Like the WF warning, no green okay light for shutter release until it dims.
In sum, this may save you the need for a cable release if using the camera handheld, but the other interlock
functions of the grip did not impress me as that useful and you cannot use the bulb setting on the
shutter (95% of my shots). Of course, this test assumes I had a working model. I did not test with flash.