Impressive southpaws set to bring the heat Thursday

With all due respect to Justin Verlander vs. Danny Duffy and Aaron Nola vs. J.A. Happ, Thursday's most interesting pitching matchup is going to happen at 1:10 p.m. ET at Tropicana Field, where two lefties with a combined four starts this year face off. That's right, Blake Snell against James Paxton isn't just a good pairing. It's a must-watch match-up.
If that sounds like high praise, well, they've earned it. Both young lefties come armed with fastballs unlike any other starter in baseball. Let's explain why.
  Because Snell has elite fastball spin and movement.
We got our first look at Snell, the Rays' No. 1 prospect per MLBPipeline.com, in April, when he came up for his Major League debut and allowed just one run in five innings at Yankee Stadium. While averaging 94.6 mph on his fastball was impressive -- it gives him one of the 10 hardest lefty fastballs in the Majors -- it was really the spin and movement on the pitch that left an impression.
Snell averaged 2,501 rpm on his fastball, and that's a big deal. Among 102 lefties to throw at least 50 four-seam fastballs this year, that's the fourth highest, behind only Aroldis Chapman, Ian Krol and Drew Pomeranz. (As we investigated recently, Pomeranz's breakout year has been due to pairing a very good curve with that high-spin fastball.) Since the Major League average is 2,258 rpm, that's considerably above average.
As we've learned in the Statcast™ era, high-spin fastballs lead to swinging strikes and popups. All pitches sink somewhat due to gravity, but high amounts of spin let fastballs defy gravity slightly longer (often known as a "rising" effect), which can fool hitters into swinging under it. Spin is a big part of movement, so keep that in mind when you see where Snell ranks on this list of vertical movement leaders:
Four-seam fastball vertical movement leaders, 2016

1. Snell, 13.6 inches
   
2. Marco Estrada, 12.9 inches
   
3. Chris Young, 12.6 inches
   Gif: Blake Snell's strikeouts
   "Vertical movement" doesn't mean that the pitch is going "up." It's showing how much higher (or lower) the pitch is than a generic pitch without movement, and these names make sense -- Estrada and Young both succeed in large part because of their spin and movement. Of course, Estrada throws his fastball 89 mph and Young only 88.2 mph. Snell's mix of spin, movement and velocity from the left side is unique for a starter.
     Because Paxton has turned himself into a flamethrower.
   Not to be outdone, Paxton has taken a fastball that's sat in the 94 mph range for his first few stints in the big leagues and… well, just look at his velocity chart:

James Paxton's velocity increase is still the best thing. pic.twitter.com/qsSGQEh4Q8

— Mike Petriello (@mike_petriello) June 15, 2016

That's an average of 97.5 mph, which is the second hardest of those 102 lefties, behind only Chapman. All told, Paxton has touched 99 mph 28 times this year, again putting him second behind Chapman -- except, don't forget, that Chapman is a reliever. Going back to 2008, five lefties have thrown at least 10 99-mph pitches, but only Paxton and David Price have done it as starters.
So what changed? Paxton's vertical release point has dropped by nearly half a foot, giving him what pitching coaches termed his "natural arm slot." You can see the difference in this side-by-side comparison. At left, the "old Paxton" was throwing more over the top, while this year's Paxton is throwing lower and harder.
Gif: James Paxton side by side split screen
Does a huge jump in fastball velocity matter? Well, it's certainly hard to make contact with the pitch. Paxton's fastball is getting a 28 percent strikeout rate this year, as compared to nine percent the past two years. It's not perfect, yet -- he's allowed a .578 slugging percentage against on it, so the pitch is definitely a work in progress. Still, it's difficult to see that huge velocity jump and not be impressed.
The point being, if you happen to be in the vicinity of St. Petersburg on Thursday afternoon, skip work, skip school, whatever you must -- just be there. With these two lefty fastballs, you won't be disappointed.. 

Read more:

• Seattle Mariners
• Tampa Bay Rays
• James Paxton
• Blake Snell