I need someone to clarify some things for me about automatic transmissions. . .

I currently drive an manual 240. I learned to drive on this car five years ago, and continue to drive it to this day. I rarely, if ever, drive my parents' cars (automatic 96 accord and 98 camry). When i do drive these cars, and i need more power, i floor it, the car downshifts, and everything is ok. What boggles my mind is the behavior of some friends of mine with their automatics. When we're going up a hill (or something of that nature that requires more power) they tend to manually shift their automatic from D4 to D3. Also, my dad continues to tell me the turning off the overdrive in his car will make us accelerate faster.

From the way i understand things, these ideas make no sense. When starting from a stop, the car starts in first gear. if you're giving it a lot of gas, it will usually shift right before redline, regardless of whether or not overdrive is engaged. And if you're trying to accelerate (to pass, for example) the car will pick the lowest gear that can be used at that engine RPM, and use it to redline, then shift. So nothing extraordinary is being done in either case.

From this, i concluded that all they are really doing is decreasing the time required for the trans. to shift into that lowest possible gear, thereby gaining a fraction of a second.

i can only think of two scenarios when I would shift in a manner such as this. The first would be if i was going through the mountains and i got sick of the transmission hunting between 3rd and 4th. The other would be if i was hauling a trailer and I wanted to take advantage of engine braking. Is my reasoning correct? Are these people out of their minds? Or am I just thinking too much like a manual transmission?