What does this button on the shifter do???

There is button on the side of the shifter knob. Normally they are to allow shifting, but the XJ has a switch on the brakes that won't let you shift w/o the brake on. So what is the button for?! I don't need to push it to shift Pto D or 3 or 1-2 So what is it? I hear people talking about locking the trans. after it downshifts on a hill. IS this button what "locks" the gear in, or am I asking a really dumb question? Is the button just to look cool?

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The button locks out overdrive. Makes it a 3 speed transmission, for towing or speed control downhill in the mountains.

I also used mine around town where speeds were below 50mph.