Commanding off speed pitches in the Go Zone behind in the count will expand the number of pitches a hitter has to protect against.  Hitters don't know what to expect when you are throwing two or three pitches in the Go Zone, this makes timing more difficult.  When a hitter knows you can't command off speed in the Go Zone they will expect fastballs when ahead in the count, timing will likely be good, and a more precise Lokation will be required to get the out.

If you can't command an off speed pitch in the Go Zone, then hitters can eliminate those from pitches they will swing at.  When you get behind in the count 1-0, 2-0, 3-0, 2-1, 3-1 (hitter's/ fastball counts) and can't throw off-speed pitches for strikes, then you are limited to throwing fastballs behind in the count.  The fastball is exactly what the hitter is looking for while they are ahead in the count.  This will decrease pitch possibilities and make the fastball look slower (decreased perceived velocity) because the hitter is expecting it.  

The fewer pitch possibilities a hitter must think about, the more likely he will expect or sit on the pitch he is going to see. This helps a hitter's timing and pitch recognition. This is why it is important to have the ability to throw the change-up or curveball in the Go Zone when even (0-0, 1-1, 2-2, 3-2) or behind in the count. 

Ideally, a change-up looks more like a fastball's flight path, which is why we recommend mastering the change up in the Go Zone BEFORE the curveball. Why not master the pitch that looks the most like a fastball and doesn’t put extra stress on your arm first?  On average hitters do the most damage when you are behind in the count.  It’s not a coincidence that almost every starting pitcher in the Big Leagues has an off speed pitch they can throw in the Go Zone in any count.  

Older pitchers should work on throwing breaking pitches at two different speeds so the hitter can’t immediately judge the speed when he recognizes the breaking ball out of your hand.  A "get me over" breaking ball with less velocity is good to throw even or behind in the count because the hitter is probably expecting a fastball.  It doesn't need the velocity or sharp break in a fastball count.  But throwing that same "get me over" breaking ball ahead in the count when the hitter is expecting off speed, they would have a better chance at solid contact.

The slower type breaking ball is also a good backdoor pitch Lokated in the arm-side third of the Go Zone when facing an opposite-handed batter. It should start up and away from the hitter in the Purpose-Zone and end in the Go or Chase Zone away from the hitter. The faster breaking ball will generally be a sharper, later break starting in the Danger or Go Zone and breaking towards a Chase Zone or an opposite handed batters back foot, used more often with 2 strikes. 

There is no need to paint the corners with an off-speed pitch when you are even or behind in the count.  This translates to generally targeting the 2 or 4 Zones when throwing an off-speed pitch while even or behind in the count.  Aiming for the middle of the plate and down (2 or 4 zones) gives you room for error when you have to throw a strike because if you miss six inches to the right or left it's still a strike.  As long as the pitch is below the top of the Go Zone with good arm-speed, a downhill plane, and correct velocity, the hitters chance for solid contact decreases. 

Generally when throwing off speed pitches behind in the count, you can get away with less precise locations.  If you threw a change up in the Danger Zone in a 2-0 count, it would be less likely to get hit than a fastball in the same area, especially if the first two pitches were fastballs.

In another example, if you are behind in the count 2-0 and throwing a fastball you might have it throw a fastball to the 1 or 5 to have a good chance to get an out.  But if you were to throw a change up in that same count you could target a bigger area such as 1-4 to get the same result because the hitter it most likely expecting a fastball while they are ahead in the count.

Throwing off speed pitches while behind in the count will allow you to throw at bigger areas because hitters are looking for the fastball.  You need better Lokation on the pitch when the hitter is expecting it or has good timing.  You don't need the best Lokation when the hitter does not know what to expect or timing is bad.  Good timing makes Lokation more important and bad timing allows for less precise Lokation.  Commanding off speed pitches in the Go Zone will disrupt timing every time.