Gears, Clutches and Bands. Planetary Gearsets. Simple planetary gearsets contain three components Internal (ring) gear / (annulus gear) Planet gears (and carrier) Sun gear One component will be the drive member, one the driven, and one will be held (except direct drive and neutral).
When the transmission is in low gear, the band is holding the high gear drum stationary which also means that sun gear S1 (flange gear) is stationary. Input is provided through sun gear S2 (center gear). This input is transmitted to pinion gear P2/P3 then to pinion gear P1 (auxiliary gear). Since S1 is being held, P1 is driven around S1 along with the carrier and output shaft.