The at Bat Starts on Deck
The next hitter, timing the pitcher from the on-deck circle.
How it actually works. On deck you have a clear view of the pitcher throwing to the guy ahead of you. Time his fastball with your load and your stride, like you are hitting. Pick up his arm slot, because that is where every pitch is going to come out of. Watch what he throws in which counts and what his offspeed looks like out of the hand. By the time you walk up, you should already know what you are hunting.
Why it is done, and when it is wrong. Hitting is timing, and you cannot afford to be late. The pitcher gets warmups. Your warmup is the on deck circle, and it is the only live look you get before he is throwing at you. That is why the hitters who grind good at bats are usually locked in before they ever leave the circle, and the ones who get beat by the first pitch were usually talking to the dugout when it was thrown.
What people get wrong about it. The most common mistake at every level is simple distraction: chatting with the dugout, looking in the stands, swinging a donut without watching a single pitch. Nothing about the swing is wrong in that at bat. The hitter just showed up late.
At Little League and travel ball. Young hitters rarely have any idea what a pitcher throws, and nobody has a scouting report. On deck is the scouting report. Teach a kid to watch two or three pitches with his timing going and he will be on time far more often.
Batter's Eye: look up anything in baseball