snap on impact operation question

[bdog]

Just picked up a 3100 snap on impact off ebay.

The power settings and the reverse on the back are like normal, but what does the hole with the open and shut lever by the trigger do?

I have owned a half dozen impacts and never seen one of these before.

 

[bdog]

Any have an idea what it does?

 

[krusty the clown]

it shuts the power down by restricting exhaust flow for low torque use.