When is a 96.4 mph fastball just as fast as one that averages 97.3 mph? Trust us, it's not a trick question, so long as you remember that the requirement that a pitcher deliver the ball from 60 feet, 6 inches from home plate is more of a suggestion than a rule. The answer: When one of those fastballs (Jose Fernandez) is being delivered 7 inches farther off the pitching rubber than the other (Nathan Eovaldi).
We treat velocity as something of a constant, as though everyone is throwing from the same spot, all the while quietly knowing that a pitcher's release point ensures that's not actually true. We can do better. Let's do better.
Since the entire point of desiring hot heat is to decrease the amount of time that a hitter has to make a decision on whether to swing and where, why don't we just show that? That's something we can now do with Statcast™, and it's particularly relevant in light of their early-season matchup in Queens. While you probably shouldn't need more than a pulse to get excited over the breathtaking thought of seeing Noah Syndergaard facing Fernandez, knowing now that we're seeing the pair of starters who gave hitters the least time to react makes it even more fascinating.
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Let's explain. We learned last year just how important extension can be, since we saw a spread of several feet when it came to where the ball was coming out. For example, Statcast™ legend Carter Capps and his unique hop off the mound had an extension of 8 feet, 4 inches, while former Dodger Joel Peralta managed only 4 feet, 11 inches. Put another way, Capps had to throw the ball just over 52 feet, while Peralta had to get the pitch well over 55 feet. That's how Peralta lost nearly 3 mph on his perceived velocity, dropping from 90.2 mph out of the hand to 87.6 mph when it reached the hitter, and how Capps added more than 3 mph, jumping from 98.1 mph to 101.7 mph.
It's clearly important. So if you combine pitch velocity and extension point, what you can do is output "plate time," which is as simple as it sounds -- it's the time, measured in seconds, from a pitcher's release point to when the ball reaches home plate, and arguably more important than pure velocity alone.
Looking at the top 10 starters (on four-seamers) from 2015:

That Syndergaard appears is hardly a shock -- after all, he lit up radar guns in his first start this year and his 97.4 mph average fastball led starting pitchers in 2015. Of course, he's also listed as 6-foot-6, and his extension of just under 7 feet was third among starters, behind only Michael Wacha and John Lamb. That helped Syndergaard's perceived velocity look like 98.4 mph, because a hard pitch thrown from closer to the plate will, of course, seem harder.
Fernandez's return from Tommy John surgery couldn't have gone better, as his 96.4 mph four-seamer was the fourth hardest of any starter, behind Syndergaard, Eovaldi and Yordano Ventura. His extension of 6 1/2 feet isn't quite what Syndergaard gets, but it's still above average, and that combination is how his plate time is just under four-tenths of a second. Good luck trying to hit that.
Gif: Fernandez 3Ks in first frame
By comparison, pitchers without elite velocity and/or with below-average extension simply let the hitter have more time. A sample of more than 600 pitchers who threw four-seamers in 2015 revealed an average plate time of 0.41 seconds. Dan Haren, for example, averaged 86.8 mph on his fastball and under 5 1/2 feet of extension, leaving the hitter with 0.443 seconds. 
It may not sound like much of a difference between elite and below-average. In real-world terms, it may not be much. But in baseball terms, that fraction of a second can mean everything. Scientific studies have shown that it takes, at best, 0.215 seconds for a hitter to look, think, decide and act -- that's begin to swing, not fully swing -- which is more than half the time the ball is actually in the air against baseball's best. Seconds, or more accurately, tiny fractions of a second, count for a lot.
When we see Syndergaard against Fernandez on Tuesday, we'll likely be focusing on velocity numbers like 96 or 99, or, if we're really lucky, over 100. But we'll know that for the hitter, that's not what's important. The important number is just over one-third of a second, which is what they'll have at their disposal to make good contact against a fastball. It's a wonder that anyone ever gets a hit off anyone.