When the pressure drops, springs lead to the clutch to release. Evenly spaced ridges, termed splines, line the inside and out of doors with the clutch to lock in to the gears as well as the clutch housing. You may read more about these clutches in How Automatic Transmissions Function. https://clutchplate53014.yomoblog.com/39537773/the-single-best-strategy-to-use-for-clutch-plate