It looked to me like an accumulation of punches that stopped Hearns, namely that 25 or so punch combination he nailed him with in round 13... he did catch Hearns with 1 solid right hand in round 6 that shook Hearns enough for Leonard to dominate round 7 & just edge round 8 but Hearns was back on his rounds 1-5 form from round 9 til 13 were it was both the tiredness and Leonard`s punches that wore him out. I`m going strictly from memory here though, I haven`t watched the fight in full for more than a year, I used to watch it regularly though. :good
Sharp puncher vs a more solid one, with a slower delivery system. If both hit a guarded man, Hagler will make the bigger dent. Yet Ray could blitz people with his combination of timing, handspeed, offensive technique and land perfectly.