To go a little further on what other people have written about "detecting current."

The brake passes a small current through the saw blade at all times. Your skin (or, well, a hot dog) is conductive. The brake monitors the resistance of the circuit and when the resistance increases significantly (i.e., your skin makes contact with the blade), the brake is triggered.

As /u/Hurlaroid points out, it also means that the brake will get triggered by other materials that can significantly increase the resistance in the circuit such as wet wood or metal. The brake system typically has a bypass mode which will disable the brake as well as provides an indicator as to whether or not the brake would have been triggered while cutting a conductive material.