Pitch Sequencing
The order of pitches that sets up the out.
How it actually works. A sequence works on eye level, on speed, or on the hitter's expectation, and the best ones work on all three. Fastball up then curveball down attacks eye level. Fastball then changeup attacks timing while looking identical out of the hand. A pitch off the plate that a hitter chases moves his idea of the strike zone for the rest of the at bat.
Why it is done, and when it is wrong. The single most useful idea here is that a pitch is not good or bad on its own, it is good or bad relative to what came before it. A ninety-two mile an hour fastball after a changeup is a different pitch to the hitter's eye than the same fastball after another fastball, and the hitter's swing is built on an expectation formed two pitches ago. That is what a catcher is really doing when he calls a game: he is managing what the hitter believes.
What people get wrong about it. Predictability is the enemy and pitchers fall into patterns without noticing. First pitch fastball, put-away slider, and every good hitter in the league sits on the slider. The counter to a hitter who is sitting is not a better pitch, it is the same pitch in an order he did not expect.
At Little League and travel ball. Below about fourteen the sequence matters far less than location and strikes, and teaching a young pitcher to think about setting hitters up before he can locate a fastball reliably slows him down. The one exception worth teaching early is changing speeds, because it works at every level and it protects the arm.
Batter's Eye: look up anything in baseball