The main parameters (plural) are to increase the load used over a period of time, to use frequent bouts rather than infrequent bouts, decondition when necessary and to use mechanical load as a means to cause muscular adaptation. The main idea of HST has nothing to do with how many reps you are doing. Using the two week blocks of 15s, 10s and 5s is simply a convenient way to implement the principles of HST. You could implement them a variety of other ways as long as you are using mechanical load to cause growth, lifting frequently and progressing the load used over time.