The grey area under the Chase Zones (4, 5, 6) and the Purpose Zones (7, 8) is called the Under Zone or the U Zone. It doesn't have outlined dimensions like the rest of the Zones but it still very important area to understand and use. Often times when a pitch is swung at in the U Zone the results are a ground ball or a swing and a miss, but the key is to realize how to get swings at pitches in the U Zone.
The U Zone shares many of the same qualities as the Chase Zone 4 other than you will rarely get a called strike call from it. It is a safe place to miss when you are targeting Go (1, 2, 3) or Chase Zones. It is almost as important to miss in the Under Zone as it is to hit the Zones you are targeting. The U Zone guarantees a downhill plane on the ball and makes it difficult for the hitter to match the bat plane with the ball plane, this will reduce extra base hits and home runs. Damage control is a huge component of the Lokator System.
Hitters who have trouble with the breaking ball are very good candidates to fish for the curveball or slider in the Under Zone, especially when they are behind in the count looking to protect the plate. Most breaking pitches that end in the U Zone have good tilt or depth, meaning there is most likely vertical movement on the pitch as it crosses through the hitting area. This will always make it harder for the hitter to make solid contact.
When a hitter takes a fastball in the Go Zone they recognize what the pitch looks like while the ball is in flight. If you can follow the fastball with an off speed pitch starting in the same spot of the Go Zone, they will be more likely to swing after tracking the flight of the previous fastball. Whether or not you use a breaking ball or a change up, the off speed pitch starting in the Go Zone will most likely end up in the U Zone instead of Go Zone, making the pitch more likely to be swung at. To get swinging strikes in the U Zone it is crucial to command the Go Zone with your fastball.
For older pitchers who throw a split finger fastball, the U Zone can be the most important strikeout area of the Lokator. A splitter that starts in the bottom of the Go Zone and has a late tumbling action towards the U Zone can promote a ton of strikeouts. Pitchers with split finger fastballs should also concentrate on commanding their fastball in all three sections of the Go Zone and following them with splitters in the same slot finishing in the U Zone.