By the middle of the 2016 MLB playoffs, it was all but a foregone conclusion that Andrew Miller's usage pattern and dominance during the Cleveland Indians' postseason run would change the way that teams build and deploy their pitching staffs. With Terry Francona's rotation already decimated by late-season injuries, the Tribe skipper was quick to pull his starters at the first sign of trouble, and Miller was often the first man up in those situations. Half of Miller's playoff appearances came in the sixth inning or earlier, and all of them lasted more than an inning. This seemed like a dramatic shift in usage pattern that would likely affect roster construction and managerial decision-making for other teams as they might try to copy this strategy (albeit with much less effective arms than Miller).
However, as far as using relievers earlier in games, the leaguewide revolution had really already long since started. As writers deemed each successive year in the early 2010s the Year of the Pitcher, MLB-wide ERA was going down and -- as one might expect if pitchers are performing well -- the average length of starts was going up.
This trend came to an abrupt halt in 2015, when the average length of a start dropped from 6.0 innings to 5.8 and the number of relief appearances in the league as a whole jumped by almost 650 games. This could have been a copycat trend in response to the Kansas City Royals' World Series run in 2014, in which they rode a dominant and deep bullpen and came within one base of winning a title. Their championship in 2015 with the same core of players gave even more credibility to their roster construction, and the Indians' 2016 performance has solidified the new reality that deep bullpens can carry a team.
Regardless of what might have sparked this change in bullpen usage in the past few years, it is clear that teams are using their bullpens differently. But how is it different, and perhaps more importantly, when are they using their pens differently?