The power button makes the car hold onto lower gears until about 5000 rpm instead of changing around 3000. Hold will keep it in whatever gear (1 2 or 3) you have selected with the shift lever.
My GTS was auto, and I found that I didn't need them too much; partially because I just changed gear with the shift lever anyway (prolly not too good for the trans), and partially because after I reset the ECU, it figured out that I liked it to rev up to 7500....so it did....regularly...without using the power button even.
