Your camera meter is set to meter an 'average' scene which has a mixture of brightness and darkness which equates to 18% grey. If your scene is not average, the meter can be fooled and give you under or over exposure. In such a scenario, you simply put the grey card in the same light as on the subject and meter the card. Then adjust your aperture and shutter speed to match the reading. You will have to be in manual mode to do that and for the settings not to change. Or in auto mode that can lock that meter reading. Shoot and check your image or histogram that the bright and dark areas are within the graph. If not, adjust your settings to suit or edit in a photoeditor later.

I YouTubed it and found a video showing what you're explaining. I thought it was pretty cool idea, but didn't have a grey card.

I fired up my favorite word processor and formatted the page to have a gray background. I couldn't do 18%, so I settled for the 20% gray and printed a page. B&W LaserJets do gray real well.

I had the Canon T1i dial on P and took a couple shots. I made use of AE Lock rather than putting it in M. I was pleasantly surprised with the results.

Again I learn something here. Maybe someday I'll learn to keep my shadow out of the pic.