On the surface Mariano Rivera and Mark DiFelice should be two of the most similar pitchers in the game. They are both right-handed relief pitchers who throw a cutters almost exclusively, over 90% of the time. No other pitcher throws the cutter that often, and only a handful of pitchers throw any one pitch close to as often as Rivera and DiFelice throw their cutters. But there is a startling difference between the two: DiFelice has one of the largest platoon splits of any pitcher, while Rivera has a reverse platoon split. Two pitchers throwing the same type of pitch almost exclusively and they are at opposite ends of the platoon split spectrum. Obviously there must be something very different about their cutters.
The most obvious difference is velocity. Rivera throws one of the fastest cutters in the game working in the low 90s, while DiFelice one of the slowest working in the low 80s. I am not sure how velocity influences the platoon split.
The movement of the two cutters is also different.
Both cutters have positive horizontal movement (tailing away from RHBs), which is uncommon for cutters. There is some overlap, but, generally, Rivera’s cutter has more ‘rise’ and slightly more horizontal tail.
Here is a breakdown of their cutters by some different metrics against RHBs and LHBs.
+-----------------+--------+--------+
| Rivera Cutter | RHB | LHB |
+-----------------+--------+--------+
| Run Value | -0.018 | -0.032 |
| In Zone | 0.513 | 0.508 |
| O-Swing Rate | 0.360 | 0.373 |
| Whiff Rate | 0.260 | 0.188 |
| Pop Out per BIP | 0.123 | 0.133 |
| BABIP | 0.299 | 0.245 |
+-----------------+--------+--------+
Rivera’s cutter is great against lefties and righties, and here you can see its incredible reverse platoon split (the run value is the average change in run expectancy after the pitch, so negative is good for the pitcher). Interesting the reverse split does not come from strikeout or walk rates (Rivera strikes out more RHBs and walks about the same), but from balls in play. Somehow balls in play off his cutter from lefties have a higher pop out rate and much lower BABIP than from righties.
+-----------------+--------+--------+
| DiFelice Cutter | RHB | LHB |
+-----------------+--------+--------+
| Run Value | -0.031 | 0.015 |
| In Zone | 0.550 | 0.550 |
| O-Swing Rate | 0.383 | 0.330 |
| Whiff Rate | 0.373 | 0.216 |
| Pop Out per BIP | 0.188 | 0.077 |
| BABIP | 0.200 | 0.333 |
+-----------------+--------+--------+
DiFelice’s cutter against righties is amazing: huge o-swing (percent of pitches out of the zone swung at), whiff (percent of swung at pitches that are missed) and pop-out rates. But unlike Rivera things fall apart against lefties, with each of these rates dropping dramatically.
Next I checked the location of their cutters in the strike zone versus right-handed and left-handed batters.
I think this is the key. Rivera works both edges of the strike zone against both lefties and righties. Since he can routinely place pitches on either edge his cutter can be successful against both. DiFelice, on the other hand, pounds his ‘sinking’ cutter down-and-away against righties. It seems that because of his cutter’s ‘sinking’ and tailing away movement (compared to a normal fastball) righties routinely swing at these pitches out of the zone. And even if they make contact the pitches are all down-and-away where most hitters can generate little power. Lefties, though, could generate tons of power from this down-and-in location and it looks like he cannot locate the pitch down-and-away against lefties. Instead most of his pitches to lefties are in the heart of the plate, not a good place for a low-80s cutter.
So it looks like the major difference is the ability to locate coupled with speed. Rivera can hit both edges routinely against both righties and lefties with his blazing cutter and somehow the pitch depresses lefties BABIP, which results in his reverse platoon splits. DiFelice can routinely locate the pitch down-and-away to righties, but has no place to go against lefties other than right down the middle.
