ROCKY: A southpaw throws your timin' off, see? Other guys, it throws their timing off, it makes 'em look awkward. Nobody wants to look awkward.
Rocky's left-handedness gives him an advantage in the ring, but in boxing, advantages are often seen as disadvantages. Most men want a fair fight.
MICKEY: Marciano... He had the same problem and this string cured it.
Mickey knows he can appeal to Rocky's competitive nature by name-dropping his idol, Rocky Marciano.
MICKEY: The idea is that if you can move and you can hit without breakin' the string, you got balance. You become a dangerous person.
This is a boxing movie, so we need to see Rocky training for the competition. Most of the time, he trains in montage, but this is a unique trick Mickey does to help Rocky with his balance.
MICKEY: Like the guy says, you're gonna eat lightning and you're gonna crap thunder!
Out of context, this sounds like a terrible idea. But in context, it's just Mickey telling Rocky how fierce he's going to be in the ring.
MICKEY: Go for the ribs. Don't let that bastard breathe.
In order to have a competitive edge, Rocky needs to take advice. This is one instance where he actually does it, and it helps him gain the upper hand in the fight.
DUKE: He doesn't know it's a damn show. He thinks it's a damn fight.
There were many boxing scandals over the years. Now, this fight isn't rigged, but Rocky isn't supposed to win. However, Rocky is a competitive man, and he isn't going down without a fight just because Creed's narrative wants him to.