Same here, if you go to practice pitching and take someone fairly accurate you can do whatever you want with the left stick after locking in and it seems to have no affect.
I am not a great player but in pitching when you learn when to release (right before the vibration) the ball goes if aimed to the same spot fairly much there. Of course the players rating has an affect.
I just threw 75 fastballs with Jon Lester (because I know whento release x with him, he taps the rubber right before the max pitch vibration) all were consitant regardless of left stick during windup. Then I threw 40 of his sweeping curves, they are less consistant of course but the location variation did not seem related to holding the left stick down and right. When holding it down and right sometimes he missed out of the zone to that area and other times it was someplace else.
Last of all I threw 50 cutters and they consistently were within one or so squares of where I aimed them. The left stick had absolutely no affect on them. I throw the pitch often enough in franchise to know for sure. My typical way is to aim and as soon as motion starts release left stick. I tried aiming holding until releasing x and tried aiming and holding throughout the motion. Tried releasing with x then using left stick again. I always aimed low and away and the used left stick to try to go further low and away. In my opinion I hit the mark (excluding a few pitches I released late and overthrew) and they went random places not seeming to be affected by left stick.
I could be wrong but this is what happened for me.
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