Pitchers are good these days. Like, really good. And while fastball velocity rules the day, the nastiness of off-speed pitches is no slouch, either.
One trend we've seen over the past couple of decades has been the use of the backdoor breaking pitch. For clarity's sake, let's first define what "backdoor" means.
You can throw a backdoor breaking pitch only to an opposite-handed hitter. That means left-handed pitchers can throw backdoor breaking pitches only to right-handed batters, and vice-versa. If you hear or read some describing a left on left or right on right backdoor breaking pitch they're using the terminology improperly.
Let's use the right-handed pitcher versus left-handed hitter example. A backdoor slider, curveball or cutter from the right-handed pitcher starts away, off the plate, and moves in to a left-handed hitter. When executed properly it is a ball almost the entire way but catches no more than the outer third of the plate at the very end. That is sometimes also referred to as a ball-to-strike pitch.
The backdoor breaking pitch that finishes in the zone is ideal when looking for a called strike, especially strike three after previously setting a hitter up with something hard inside. It is also good just below the strike zone when looking for a swing and a miss late in the count.
The backdoor breaking pitch is not a great choice when a pitcher is behind in the count and needs a strike. Reason being that the backdoor breaking pitch offers little room for error and without plus command it can be problematic. If you're throwing that pitch in, say, a 1-0 count you need it for a strike. If you can put it on the corner that's great, but the reality is behind in the count you're probably not shooting for the corner, you're trying to catch more of the plate.
If you miss over the plate with a backdoor breaking pitch it comes right into a hitter's strength, and the results can be disasterous. Notice the count and watch the catcher's glove....