Well the weak link when you bench is your triceps your tris often fail before your chest. So the principle of pre exhaust is to significantly fatigue, in this case your chest, with flyes before moving straight on to the bench and taking your chest to complete failure.

It is about eliminating the weak links, being those muscles that will fail before your target muscles do allowing your target muscles to go to failure.

Check out Arthur Jones and the Colorado experiment along with the training of Mike Mentzer, Casey Viator, Sergio Oliva and Dorian Yates for more information