What exactly is the battery for in the Letus35 adapter?

[myCharlie]

I have the Letus35mm adapter. I was wondering what the two double AA battery do. I accidentally took some footage without the pushing the red button to turn on the adapter. I don't see any difference between having pushed the red button to turn on or off. So what's it really for?

 

[Barry_Green]

It makes the ground glass vibrate, which makes the grain of the ground glass disappear.

 

[myCharlie]

I see. No wonder when press the button, I can feel the vibration.

Thanks!