When you're on the PS3 side, you go to System Settings - OtherOS Settings and tell it to boot into OtherOS. When you're on the OtherOS, you have a bootloader (usually kboot or petitboot). The bootloader gives you a way to boot back into PS3 mode (or you can hold the power button, but that resets your settings, so don't do that unless you have to).
I suggest using petitboot - it allows you to boot other things using the mouse. It's a nice graphical display whereas kboot is command line oriented.
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