The Cylinder Heads are electrically charged to create a magnetic field in which a chemical with charged particles is added.  Under ultraviolet light you can see cracks that were otherwise undetectable  by the naked eye,

Many times crankshafts have stress or heat cracks that show up on the magnaflux machine testing like a beacon.  These cranks to the naked eye would look ok to reuse.

The machine uses a high flow of current to magnetize the metal, which allows the charged particles in the solution of the magnaflux machine to 'stick' to the cracks.  Those particles react with ultraviolet light and indicate the presence of surface cracks.  This machine can also be used on Connecting Rods and other metals that hold a magnetic charge.  When the test is over, the machine is set to reverse the polarity and demagnetizes the metal.