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.
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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.