Many cars have an OD button - either on the shifter or on the dash. They work the same way. Push in the button to de-activate the highest gear. You should then see a message on the dash indicating "OD off". Do this for climbing hills, towing, or carrying heavy loads. Activating the OD off button keeps the transmission from shifting into the highest gear but leaves the other gears unaffected. You'll achieve slightly lower fuel economy but the advantage is you will not be asking your engine/transmission to operate within a rpm range that is too low for a heavy load.
The power button is different because it changes the shifts through the entire range. It is used for more aggressive driving - but at the expense of some fuel economy. If you need a more "sporty" drive, use the power button (but use a little more fuel). I like these power buttons because they give you the choice to allow the transmission to shift automatically at higher rpms - and without it, many ATs shift at too low rpms and would require manually shifting the AT to keep from lugging the engine.
Many pickups and SUVs combine these two functions into a single button called "tow/haul mode" that changes shift points while also turning off the OD.
You can buy an OEM owner's manual cheaply on ebay. For $200, you should be getting a full paper shop-service manual, which is more than you need if you are not doing rebuilds, etc.