I am going to quote directly from the Fielding Bible Volume III on the changes:
1) Timer Plus/Minus – This is the most important new addition to the DRS System. In 2009, Baseball Info Solutions started timing every batted ball from the moment it was hit to the moment of the resolution of the play (hit or out). By using the timer, we have taken some key subjective elements of the system and made them objective. For example, we used to measure the velocity of a batted ball with an estimate, calling it softly hit, hard hit, or medium. Now, we know the exact velocity of every batted ball. We have added this tour cornerstone methodology, the Plus/Minus System.
2) Good Plays and Misplays – This is a brand new methodology. When a player makes a play that is better than the norm, we want to give him more credit. Or if it’s worse, we want to take credit away. We have 28 categories of Good Plays and 54 categories of Misplays. We have converted the ones that don’t overlap with our other methodologies to Runs Saved estimates. With this system we especially made great strides in measuring catcher defense.
3) Outfielder Arms – Here we use the new batted ball timer to upgrade our system to measure the impact that outfielders make defensively with their throwing arms. We also now use a more precise location of batted balls to enhance this methodology.
4) Double Plays – We’ve upgraded our system to measure how good infielders are at starting and turning the double play by doing something similar to what we do with our Outfield Arms technique: measuring the location and the velocity of the batted ball in a double play situation to determine how difficult the play is to turn into a double play.
5) Bunts – Same thing here. We now include the location of bunts among other criteria to better evaluate how fielders make plays on bunts.
6) Centering – The intention of Defensive Runs Saved is that zero is average. If a player has a positive number of runs saved, it’s an above-average performance. If he has a negative number of runs saved, it’s below average. As we’ve added on to the system over the years, we veered away from zero a bit. To get back on track, we took every methodology and “centered” it at zero.
David Appelman is the creator of FanGraphs.