The light can be switched on or off by operation of the stem on the right of the steering wheel, the same control that operates the windshield wipers.
I can't detect any difference whether it is on or off.
That light indicates the Exhaust Brake feature is active and will work to slow the vehicle when your foot is taken off the accelerator. The picture is meant to be a throttle plate in the closed position (the horizontal line with the dot in the middle) and air being stopped by the plate (the vertical lines).
Here is a question on another site that describes it:
It has 2 uses. In the winter you flip a switch and the hot exhaust goes back through the engine to warm it up faster. When driving down the road you take your foot off the throttle and a switch kicks in to redirect the exhaust to help slow the truck down. Saves on the brakes if you not a speedster.
And this is a product image of the turn signal switch for that truck from Amazon whose description says:
List Price Is Over Than $300.00 It Has The Exhaust Brake Switch For Diesel Engine.