Arduino UNO powered through input pin

While troubleshooting a project, I came across a very strange phenomenon. When you connect the UNO board to ground and connect 5V to any of the input pins, the board behaves virtually the same as if you powered it normally. If you would like to try this, load the blink example, connect the ground on the board to the ground on your power supply, and connect 5V to digital pin 7. The led will blink and the power light will come on.

Why does this happen? It is repeatable across multiple boards, and is repeatable across different microchips as well (A colleague reported the same phenomenon with a D/A converter chip). I'm not so much looking for a solution, but an explanation. Any help would be greatly appreciated.

well,(i geus,) inside the micro, u force the pin D7 to a 5v and the current will reach the cpu clock and the power, only difference will be the pin D7 while this is forced to be 5v but you will not notice the differnce because the pin is in use for the power..

please confirm nick

IO pins are not supposed to be driven higher than VCC +0.5V. Long term, you can cause damage to pin. Do not do this on a regular basis. Explanation - you powering the part thru its IO pin protection circuit.