The extraordinary look, which is created by injecting saline into the forehead, then pressing in the centre of the swollen area with a thumb, is a hit on the country's underground body modification scene.
The dramatic results of the two-hour treatment last just 16-24 hours, after which the saline is absorbed by the body and the forehead reduces back to its normal size.
The process is revealed on National Geographic's Taboo, where three people are seen undergoing the treatment in Tokyo.
It shows John, Marin and Scorpion having a large needle, attached to a saline drip, inserted into their foreheads.
The practitioner administering the treatment, Asami, is heard saying: 'You really need to do it the right way, to reduce the risks.'
The sensation is evidently uncomfortable. As it punctures the skin, John says: 'Ooh... That's a needle going into my head... Ooh.'