Banned & doctored
Spitball
Illegal since 1920. A pitcher wets one spot on the ball so his fingers slip off it, stripping the spin that holds a pitch up, and it dives sharply and unpredictably. It's outlawed, and any pitcher caught throwing it gets tossed.
The grip
Saliva or another slick substance is applied to one part of the ball so the fingers slip off it at release, killing the friction and backspin that normally hold a pitch up.
What it does
With friction gone, the ball drops sharply and erratically on the way to the plate instead of carrying like a normal fastball.
What it really is
Outlawed by the Joint Rules Committee on February 9, 1920, along with the other doctored deliveries; 17 active spitballers were grandfathered in and allowed to throw it for the rest of their careers, with Burleigh Grimes the last, retiring in 1934. Decades later Gaylord Perry weaponized the mere suspicion of a spitball, titling his autobiography 'Me and the Spitter' and cataloguing how he dodged detection.
Correction to ground truth: the February 9, 1920 ban PREDATES Ray Chapman's death (August 16, 1920), so the ban was not 'after' Chapman. Chapman's death followed the ban and drove the separate clean-ball reforms and accelerated full enforcement after the 1920 season — it did not cause the February vote. February 9 date confirmed via SABR ('Spitball and the End of the Deadball Era' / SABR Century); the 17-pitcher and Grimes-1934 facts confirmed via Wikipedia.
Who throws itBurleigh Grimes, Gaylord Perry, Ed Walsh
Basic file
This pitch has a sourced one-line grip and movement and an honest explanation — not yet a filed specimen with authored grip geometry and a full craft chapter. A fuller breakdown is coming. Sourced, not corrected.