Keller used his own Rapsodo device during the shutdown period to improve the spin efficiency on his fastball, Alex Stumpf of DK Pittsburgh Sports reports.
It sounds like Keller was very self-motivated to improve as a pitcher during the shutdown period, using the Rapsodo (a pitch design tool) to up his spin efficiency from the low 90s to the 95th percentile. He has a reputation for having the velocity but not the ideal movement on his fastball, which is sometimes referred to as "straight", so if Keller's fastball can miss more bats he could be primed for a breakout 2020 season. Keller said Monday that he feels "really good" so he appears primed to open the season in Pittsburgh's rotation.