What it is 
Arm Slot is the angle that the pitching arm takes through the top of the pitching motion up to the release point. It is represented by the yellow arrow on the Clockface and 3D Clockface. Arm Slot is expressed as time on a clockface when viewed from behind the pitcher. For example, a righthanded pitcher would generally have an Arm Slot between 12:00 (overhand) and 3:00 (sidearm).

Why it matters
A pitcher’s Arm Slot has a big impact on what type of spin they are able to generate. For a fastball, a higher Arm Slot will tend to generate more Backspin and thus more Vertical Movement. Pitchers with a lower Arm Slot will tend to have more Side Spin, which will create more arm side run.

When throwing a fastball, aligning the Spin Direction with the Arm Slot can be a good indicator of staying behind the ball and achieving high Spin Efficiency. If the Spin Direction is higher (closer to 12:00) than the Arm Slot, it may indicate that the pitcher is cutting the ball.

For any given pitch a consistent Arm Slot helps a pitcher achieve consistent control. Having a consistent Arm Slot across pitch types makes it more difficult for the batter to identify the pitch type at release. If the Arm Slot for a fastball and curveball are the same, it is more difficult for a batter to recognize the type of pitch thrown.

How it is used
To get a feel for what your Arm Slot looks like, try throwing each of your pitch types a few times and examining the Arm Slot for each one. Try to keep your Arm Slot consistent across all of your pitches. This will lead to a more consistent delivery, which will improve your control as well as disguise your pitches better.

A changing Arm Slot can also be a good indicator of changing mechanics over the course of a season or a career. Fatigue and other factors can cause your Arm Slot to change over time, which may lead to deterioration in performance. If a particular pitch seems to be less effective than earlier in the season, looking at the Arm Slot of pitches in your history may help you find what has changed.

Arm Slot can be very useful even when you are working on other aspects of your pitching. Spin Direction and Movement are heavily influenced by your Arm Slot, so looking at your Arm Slot provides important context when you are trying to understand them. If you are trying to figure out why you aren’t getting more ride on a fastball, it could be an issue with your delivery or your spin, or it may simply be that you have a lower Arm Slot.