Are you talking about the internal reversed biased diode in a mosfet?
In a N channel MOSFET, you have a P doped silicon substrate, and the source and drain are N doped regions within the substrate. So in normal operation, both the source and the drain are in fact diode junctions to the substrate. The source is also connected to the substrate as for the gate to work properly as the substrate has to be at the same potential as the source. There are actually some small signal MOSFETs that have the substrate connected instead to a separate fourth pin, but these are now increasingly rare, and I have never seen it done in a power MOSFET.
So in normal operation, the drain region is a reverse biased diode junction to the substrate which is connected to the source. If the drain voltage is taken below the source voltage, this diode junction is now forward biased.
Richard