Bench bleeding is done by filling up the MC reservoir with brake fluid and pressing the plunger in several times using a blunt object to remove air from the MC. Air must be bled out from the MC either before or after installation on the vehicle. Bench bleeding saves time and will also tell you if the rear MC seal is leaking. Sometimes a new or rebuilt MC can be bad right out of the box.

Here is a link to a basic "How-To" guide. I keep a set of blank MC plugs in my toolbox to install after bench bleeding. I also use these same plugs to test a suspected bad MC when isolating a brake problem.

http://www.misterfixit.com/brakbld1.htm