NVIDIA works again. Why?

Hi

My Mid-2012 Macbook Pro with Retina (15") has a dual graphic card.

- NVIDIA GT650M (default)

- Intel HD 4000

Few weeks back, I thought my NVIDIA card went "dead".

On boot up, got a blank screen. Had to close the lid and cover the Apple logo for few seconds.

On re-opening the lid, the screen light up (with the login screen). I saw this trick in Youtube.

The intent was to let the Intel HD 4000 graphic to cut in.

The system display did not show the existence of the NVIDIA card.

Sending back to Apple was too expensive.

I was planning to get it repaired at a specialist shop for half the price. They are aware of the similar Macbook (Mid 2012) with the same issue.

Today, as I was about started the Macbook Pro, and everything starts as per normal.

Nvidia graphic card (and the Intel HD 4000) is listed in the system.

Any idea why?