Here’s one possible explanation: if the pitcher on the short leash starts to look shaky, puts a few runners on, and allows a run or two, then you pull the pitcher and call in a relief pitcher. Also the bullpen needs to be ready to perform this task. As the Washington Post put it, the bullpen is put “on code red”.
So, let’s summarize. When a pitcher is on a short leash and he’s not pitching well, the manager should remove the pitcher and call for the bullpen. Brilliant! It’s such a brilliant strategy, you’d think that teams would employ this strategy in every game, no matter who is pitching.
Oh, wait. I looked it up. Teams already do this. They all have relief pitchers waiting in bullpens to come into games when starting pitchers falter.
Maybe there’s something more to the “short leash”. Maybe a manager is supposed to pull the pitcher on a “short leash” earlier than he’d pull a pitcher who’s not on a “short leash”. …