I'm guessing your talking about the overdrive button on the gear shift. If your on the highway going 65 and the tranny is locked in 5th and you hit that button, the truck will downshift into 4th gear and if your going downhill your face will hit the windshield. It is for towing. I hope this helps.
I AM NOT trying to be a smart ass here, but it's gonna sound like it.........
I giving my 2005 Tundra to my soon to be 16 year old son Daniel. 2 days ago I suggested that before he hit the road in his truck and begins his driving life that he read his owners manual. I drove up yesterday evening and the boy was outside with all four doors of "his" truck open and the hood up. I pulled up beside him and asked him what he was doing with "my" truck. He folded the back seat back down and grinningly lifted the owners manual from the back floorboard. He'd read the whole thing and was checking it out against the truck. My father-in-law used to buy a new car and then immediately sock the owners manual in the top of his closet. I asked why one day when I was changing his oil and needed some info and he had to go FIND the manual. I pointed out that when he was stuck somewhere and needed that little nugget of info that was contained in the manual stored in his closet, that he'd wish it was in the glove box. He still keeps them in the closet..........
Get a manual if you don't have one already. They are REAL handy.
I agree with David Jones completely. If you read through the manual you will learn about all kinds of features for your truck. I would ESPECIALLY recommend the 4wd mode section (if you're running 4x4). For example, the manual recommends driving the Tundra in 4wd mode a few miles a month to keep the components properly lubricated and functioning properly. That might be common sense to some of you 4x4 veterans, but I didn't know that.
Also, getting into 4-lo was not as easy as I thought it was going to be. Yeah I know, make fun of me. I had a 95 Tacoma with manual wheel locks and a shifter before this. The first time I tried using it, I thought it was broken. Fortunately, I pulled the manual out of the glove box and figured it out before calling the dealer to complain, and looking like an incompetent ass.
or when your towing something heavy and also dont wanna burn brakes.... and if you lived in socal and your going downhill on the freeway with traffic going about 10-15 mph... also doesn't burn brakes,,,