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Pluge. Whatever that is.

"PLUGE" is one of the tests:

The PLUGE (picture line-up generation equipment) pattern is used to adjust the TV's "brightness" or black level.

The inner bars are a signal level slightly lower than standard black, which causes some TVs to distort or even lose sync. The outer bars are the darkest mix of colors the NES can show.

You should adjust brightness until the outer bars are clearly visible and inner bars are not.

But how would the shark image be used with the PLUGE test?

It was the filename.

I know about the filename. But where would a shark appear in a PLUGE pattern?

Would it be a good idea to add a screensaver that begins after five minutes of inactivity, draws a bouncing logo, and plays a fanfare whenever it hits the corner?