dual power / power swap

So, my Uno takes power via barrel connector, and via USB.

Let's say I have it connected only to my computer via USB. It is now powered. What happens if I attach the DC barrel connector? Will it blow up, will it ignore, or will it gracefully move to using this power supply. Now, say I remove USB. What happens next?

If the voltage at the Vin pin is greater than 6.6V the USB power will disconnect in favor of the power from the onboard 5V regulator. If the input voltage drops below 6.6V it will switch back to USB power.