Marshall, who relied primarily on an elusive screwball, led his league in games pitched four times, saves three times, and games finished five times. He is the holder of two major league records, both of which he set in the 1974 season: most appearances (games pitched) in a season (106), and most consecutive team games with a relief appearance (13). He also holds the American League record for games pitched in a season with 90 for Minnesota in 1979. In his record-setting 1974 season, he pitched 208.3 innings, all of which came in relief appearances. He attended Michigan State University, earning three degrees, including a Ph.D. in kinesiology. In the months preceding his 1974 Cy Young season, Marshall considered retiring in order to work on his Ph.D.
Marshall teaches and advocates a pitching method he developed that he "believes could completely eradicate pitching-arm injuries." He wants pitchers to have a smooth transition. To pitch the arm has to externally rotate (hand lays back) before the ball accelerates. With traditional pitchers the thrower often lifts his elbow before his hand (also called scapula loading); this leads to the ball coming up late and the arm to externally rotate late. The elbow will be accelerated forward while the hand still goes backward, which creates enormous stress on the arm.
Marshall instead wants pitchers to externally rotate early as they swing their arm up. That means the pitcher will lift the hand before the elbow, so that the wrist faces away from the body and up, the hand is above the elbow when the front foot touches the ground, which leads to a smooth transition without a "forearm bounce," as Marshall calls it (Marshall believes this causes UCL injuries, which require Tommy John surgery). He wants to first lay back the forearm and then accelerate by rotating the body instead of bending over, in order to protect the elbow against injury.