Superradiance is a Quantum-Mechanical mechanism amplifying Electromagnetic Radiation, which can take place when the radiation strikes a rotating body. In such a case it amplifies a specific Frequency of the radiation, in effect, a resonance, that depends upon the particulars of the body and rotation. In effect, it is a type of natural laser.
Theory has it that superradiance can occur on the "surface" of rotating Black Holes, and is a means by which energy can be sapped from the black hole, slowing its rotation. This can occur spontaneously, i.e., without an external source of EMR, using EMR due to Quantum fluctuations near the event horizon (Hawking radiation).