What does the gear selector really do?
 

Reading about automatic transmissions I see that the hydraulic pressure in the transmission is what causes the transmission to shift.

So that has me wondering; when I select 2nd gear manually, what is actually going on in the transmission?

It seems to me that just because you place the transmission in a particular gear does not mean it would neccessarily have the hydraulic pressure needed to shift into that gear.

If the answer is that selecting that gear manually takes the place of the hydraulic pressure then my next question is; how could the transmission NOT go into 2nd when manually shifted there?

"Manual" operation of the gear selector does not equate to "mechanical" gear selection. All shifting is facilitated hydraulically.

That is correct. Except for the parking pawl.

It switches the valves around in the valve body and redirects the hydraulic pressure away from the passages that put pressure on the next gear. The linkage from the shifter is mechanical (a cable or rod) but that just connects it to a switch on the transmission, which is part of the valve body. From there, it's a series of hydraulic valves that controls when, what, and how to shift. That is one reason why when you shift it into gear (along with the torque converter) it takes a second to get there. It has to move the fluid into the proper valve in order to then move a piston/servo to put pressure on the proper clutch band.

Excellent, thanks!

position 2 in my car always starts in first gear but shifts to 2 at full engine rpm, I believe its done by MB to facilitate towing. But my car is NA and is a dog at slow speed - the turbo ones and gas ones could be different.