The Amazing Jonathan shows what appears to be a regular razor blade. To demonstrate that it is sharp, he cuts a playing card in half with it.
Jonathan then places the blade in his mouth and chews until it's gone. He then places five (well, four) additional blades into his mouth and swallows them. Afterwards, his mouth is filled with blood and he occasionally moans in pain.
There's no secret. The Amazing Jonathan is just f***ing insane. Try this yourself, and you'll die.