I’ve heard it called a “suicide plug”. A common use for them is back feeding power from a generator into your homes electrical panel during extended power outages.

It can technically work but comes with major safety risks such as:

• Giving yourself a nasty shock.
• Electrical fire.
• Electrocuting anyone who comes in contact with the power line, i.e. a lineman who might assume the line is de-energized.
• Blowing up your generator when the power comes back on.

The proper way to do it would be to have a transfer switch and generator plug installed. The transfer switch guarantees that when you’re running on gen power, you’re not back feeding through the transformer out to the power line.