We, at the OU, have received several phone calls asking to explain the purpose of the second cut done at Agriprocessors.
There are two reasons for this cut: kosher and commercial. The additional cut into the carotid arteries accelerates the flow of blood and its depletion from the animal and that helps facilitate a more effective, subsequent, salting (melicha). It also improves the quality of the meat by avoiding blood spotting.
The second cut into the carotid arteries, though halachically significant, need not be done by a “shochet” (as explained in Yoreh Deah, Siman 22).