back in the 70s there was no such concept as "post cycle therapy." the drugs we all know and love today were not in circulation. so as a makeshift, albeit somewhat improvised post cycle therapy for the time, many pros would taper on/ off of their steroids... which is not totally illogical. tapering on a substance like D-Bol gradually releases it into your system, whereas tapering off toward the end of your cycle over a period of time will (technically) ease the body into transitioning off the drug.
also, lots of pros only dosed on the days they worked out. so in essense, pros like arnold and others would more or less "pulse" the steroids they took. coupling that w the taper method was effective enough for the time.