In this case the professor is holding the fire extinguisher so that when it fires it exerts a force on him that does not pass close to his center of mass.  Sets him into rotation about his center of mass.

To see a (.avi, slow download) video of the "rotation" fire extinguisher demo click here.>

To see a (RealVideo, faster download) video of the "rotation" fire extinguisher demo click here.>