Sean Doolittle has thrown 4,610 pitches that have been tracked by the PITCHf/x system between 2012 and 2017, including pitches thrown in the MLB Regular Season, the MLB Postseason and Spring Training. In 2017, he has relied primarily on his Fourseam Fastball (95mph), also mixing in a Splitter (86mph) and Slider (82mph). He also rarely throws a Change (84mph).
Basic description of 2017 pitches compared to other LHP:
His fourseam fastball generates an extremely high number of swings & misses compared to other pitchers' fourseamers, is blazing fast, has good "rising" action, has less armside movement than typical and results in somewhat more flyballs compared to other pitchers' fourseamers. His splitter generates a very high amount of groundballs compared to other pitchers' splitters, has movement that suggests an extreme amount of backspin, is much firmer than usual and has slight armside fade. His slider generates more whiffs/swing compared to other pitchers' sliders, results in more flyballs compared to other pitchers' sliders and has short glove-side cut. His change (take this with a grain of salt because he's only thrown 1 of them in 2017) is basically never swung at and missed compared to other pitchers' changeups, is an extreme flyball pitch compared to other pitchers' changeups, has an extreme amount of backspin and is slightly firmer than usual.