i have just been doing research on these auto trannies as im in the middle of a swap/upgrade ,from what ive found from a factory service manual the power indicator does just what you say when you floor it it changes the shift rate and the rpms at which it shifts into each gear. the light is just telling you it has change into this different program shift mode .once you let off the accelerator it switches back to normal mode .
the manual button as per the factory manual "used to maintain the transmission in select range 2nd,3rd when going up or down steep hills,running on sand ,mud or slippery surface" took me reading the entire manual to find that out . i was hoping it would provide locked full time 4wd but i appears it doesnt ,just start you out in 2nd gear (limiting wheel slip) and then only allowing 3rd gear with no 4th or torque converter lock-up.
the other thing the power button does is flash problem codes for trans ECU if it has sensed a problem and set a trouble code.
hope this helps