With the exception of a few armbars, most submissions apply pressure through joint manipulation shoulder, elbow, ankle etc. There are tendons, ligaments etc. that stretch before breakage accurs. These are profeesional athletes that know when they are in trouble. (for the most part) there is a certain degree of professionalism that when they have an opponent in a submission and a tap occurs like clint said they will hold position and wait for the ref to break them apart.