I believe that is hard-wired to the voltage regulators on-board, the ATmega328 had no control over the power. You can use a digital output connected to a P-channel MOSFET to control the power rail (high-side switch) and a N-channel MOSFET to act as a low-side switch. If your load is very small, you could just use the digital or analog outputs directly to power things like small LEDs and such.
If the arduino could switch off it's own power, then it will kill itself because it no longer has power to turn itself back on! Like sawing off the branch you're sitting on! (It is a bit of a catch-22 situation)
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thanks for the reply. I had done the control by using the I/O pin to trigger a transistor and then run a timer to turn it off so that Arduino will not consume any power while it is off. It was perfect for my application.
Great, I'm glad you got it all figured out!