Here is the write-up I did from CD, it goes over the major points:
Button Accuracy:
The mechanic is in the form of a meter which is over the player in arc fashion (like a rainbow shape) and within the meter you have colors that transition into one another. Starting from the left side of the meter you have red then yellow which take up a little more then a third of the meter. The green section or sweet spot resides on top of the meter. The remaining portion of the meter after the green transitions back to yellow then red. So in arc fashion you have red/yellow/green/yellow/red. As with analog red will be a bad throw, yellow is questionable and green will be the most accurate result. The power portion of the mechanic gets displayed under the player with growing rings just like regular button throwing.
Your initial button press which is pressure sensitive will simultaneously bring up the throwing meter while filling rings underneath the player to determine the power of the throw. As soon as the meter activates is where it develops its personality. You will see the green section or sweet spot shift forward or backwards and either grow and shrink. This change is the player's fielding rating/attributes taking affect. A poorer fielder will see his sweet spot shrink and shift forward causing you to have to hold the button longer to hit that sweet spot. A better rated fielder will see the sweet spot shift back and possibly grow allowing you to get your throw off quicker. The meter will also change based on other factors besides a fielder's attributes, throw distance, throw difficulty, momentum etc., all will have an effect on making the mechanic have dynamic results. As with all fielding options preloading yields the best results and button accuracy is no exception. I found you need to preload even earlier with this mechanic because it takes some time to hit the sweet spot for a perfect throw. Initially I kept missing my branch point until I got used to it.
So to review you press the button, the initial press (remember pressure sensitive) will determine your strength and bring up the meter. While continuing to press you will see the sweet spot change based on ratings and the situation, a black needle will travel along all the different sections with the goal of stopping within the sweet spot. Canceling a throw occurs when you release and tap the button again.