L1+R1+L2+R2+Start+Select -- puts me back into the consoles main menu (FCMB still active and modifying the menu) (but you could probably change that to OPL - within OPL).
That "function" is part of a memory "hack" OPL does. And its important to know, that if you look up OPL compatibility lists, you will see, that for some games to work (load), the in game reset actually has to be disabled. (ICO afair is one.) You will come across such games eventually. (In that case you disable it in "per game" options for that specific game. With one of the 6 or so "hacks" OPL offers per game. (Some influence disc loading speed, one disables the patches that also patch in IGR (= in game reset)) The compatibility lists will list them when per game hacks need to be enabled (or rather disabled, by enabling a per game 'hack option' - because IGR is enabled by OPL by default).
By default L1+R1+L2+R2+Start+Select in some games would reset you back to the main screen for that game, but OPL modifies that, so it resets you back to OPL or the PS2 main menu - from any game (that doesnt crash, when thats done - in which case you usually disable it in per game options in OPL, and save that config for that game.).
From PS2 main menu, you can then launch OPL, and OPL in itself has an option to "shut down" the console (puts it into stand by only - As in doesnt flip the powersupply switch..
).
This is how I turn off the console most off the time.
But the others are correct, that on the original PS2 you were supposed to walk up to the console, and essentially long press the power/reset button (the one above eject) to shut down (or reset (short press)) the console.
As long as you arent in the middle of saving to a memory card, you should be fine.
(As per the originally intended Sony design.)
