This post was inspired by a long exchange of commentary between me and Baltimor over the game-winning home run Wily Mo Pena hit against David Purcey of the Detroit Tigers on a hanging slider.
Will try to add some pretty graphs and pictures of dots eventually...but for now you'll just have to be satisfied with this picture and my basic explanations.
The major understanding problem everyone has to deal with when trying to understand pitch f/x is the terminology. Terms like movement and spin get thrown around a lot, and people (or at least, me) just end up getting really confused over what the heck everyone is talking about. Really, movement should be broken down into two types of movement. Let's call this "real movement" and "differential movement". Real movement is the movement you see in real life, on TV, or as a batter. Real movement is caused by a huge number of factors, the most important of which is probably spin, gravity, and velocity, but it's also affected by humidity, temperature, wind and release point of the pitch. Differential movement, on the other hand, only shows the movement difference between a pitch with no spin, and the real pitch. The prime example is a fastball. A fastball has positive vertical "differential movement" because it is thrown with backspin (this is universally true), which means that compared to a pitch with zero vertical spin, the ball will "move up." The only thing differential movement measures is the effect of spin on a pitch's movement.
Fastballs vs. Sliders (Or What I Like to Call, Choose Your Own Pitching Adventure)
Now let's think about fastballs and sliders. Let's say you throw a four-seam fastball, and you throw it at 70 mph (because in this story, you're Jamie Moyer). Lo and behold, when you throw it, it looks like a curveball. You're like, wait, what's going on? I definitely threw a fastball, so why did it arc in the air, and die at the plate like the curveballs I see on TV? That's because there's not enough velocity to counteract the force of gravity, which naturally makes the pitch take an arc-like shape.
Now you magically turn into Justin Verlander and throw a 100 mph fastball. And it moves in a straight line towards the plate. There's no vertical movement at all. Why? Because the backspin on the fastball and the initial velocity balances out the force of gravity.
So then you turned into Randy Johnson and said to yourself....okay. I want a pitch that moves. But I want that pitch to be fast as hell. How do I go about doing this? If I throw a 90 mph fastball, then again, the backspin and velocity will cancel out too much of the force of gravity, and my pitch won't have any "real movement". Let's face it, major league hitters are too good...they will easily hit my fastballs if they move in a straight line towards home plate. But I don't want to throw a 70 mph fastball, which has a lot of "real movement" but no velocity, because again, major league hitters are too good and pitches with no velocity, even with a ton of "real movement", end up getting hammered.
So, you fiddle around with grips and pressure points on the baseball, and BAM, you come up with the slider, an oftentimes "spinless" pitch. The slider still has plenty of velocity (especially with guys like Randy Johnson), but it's able to generate "real movement" because without any spin, the velocity alone is not able to counteract the force of gravity. So the pitch "breaks" when it heads towards home plate.
When pitch f/x graphs show movement, what they are actually measuring is "differential movement". That's why breaking balls tend to hover around zero movement for at least one of the axis. The easiest pitch to recognize on a movement chart is the 12-6 curveball. This is because the 12-6 curveball has zero horizontal differential movement (hence, it looks like it goes from 12 o'clock to 6 o'clock) and it is the only pitch to have negative vertical differential movement, because it is the only pitch thrown with topspin, which adds onto the effects of gravity, to make the pitch break heavily in the vertical direction.
Sliders though are much more difficult to predict, because they can be pretty unique to each individual pitcher. The key lesson to take though, is that the movement graphs for pitch f/x are not capable of indicating whether a slider was "good or bad". Spinless sliders are very normal for some pitchers, because that's just how they want to throw their sliders. And spinless sliders doesn't mean the pitch doesn't move in real life, it just means the pitch didn't move any differently from any other pitch that was spinless (naturally).
And the lesson to be taken from the Wily Mo Pena home run, is that any 82 mph pitch that stays up in the zone and inside to a monster pull hitter, regardless of whether it has spin (fastball) or doesn't have spin (slider in this case), is a very very bad pitch.