If you think about it at all, for instance, you immediately recognize that Derek Jeter is capable of a good piece of hitting, while Alex Rodriguez is not. David Wright rips off good pieces of hitting like a neighborhood godfather peeling $20 bills off his bankroll to hand to kids making bodega runs for him, but Carlos Delgado has rarely, if ever, been accused of having committed a good piece of hitting. It's probably possible to work backward from the sort of player capable of a good piece of hitting to arrive at a definition of the thing itself, but it might be better to work forward — difficult as it is to extricate the player from the act.
A good piece of hitting cannot be a home run or a solidly hit double down the line. It cannot be a bouncer, bleeder, trickler, or any other sort of hit that has eyes or relies on the misadventures of the defense. It must be hit well, but not too well, and preferably it should go the opposite field.
It isn't just the character of the hit itself that defines a good piece of hitting, though, as the game situation plays its role as well. No hitting done by someone whose team is up by 10 runs will ever be said to be a good piece; the game should ideally be tight in order for the piece of hitting to be good. On the other hand, it's possible to imagine a piece hit by someone whose team is down by 10 runs being hit well, although probably only if there are no outs and he's at the front end of a rally. "That was a good piece of hitting," the announcer will say, while we watch the player taking off his gloves at first, clapping his hands, and exhorting his teammates to keep on with the charge.
I bet Marchman doesn't like sacrifice flies either.