What was the procedure behind Aztec (heart) sacrifice?

Discovery channel had a show about this not too long ago. They were trying to determine the process that the Aztecs used with the tools at hand since they would execute hundreds at a time. They first tried the typical process you would expect, they tried to open the plate and extract the heart, but the stone knifes would break fairly quickly and it would be a strenous process seperating the plate to get access to the heart cavity. As you can imagine this process was pretty slow. They finally determined that the priest would perform an vertical incision below the sternum between the lower rib cage and then reach up and extract the heart. They were able to simulate doing this process in under 5 seconds. Pretty brutal people for sure.