Clutch shift control is critical for the performance and fuel economy of automotive transmission, including both automatic and hybrid transmissions. Among all the factors that influence clutch shift control, clutch fill and clutch engagement are crucial to realize a fast and smooth clutch shift. When the clutch is not engaged, the fluid held by the centrifugal force inside of the clutch chamber, which introduces additional pressure that will affect the subsequent clutch fill and engagement processes, should be released. To realize this function, a ball capsule system is introduced and mounted on the clutch chamber. When the clutch is ready to be filled for engagement, the ball capsule needs to close quickly and remain closed until the clutch is disengaged. It's also desirable to have appropriate closing velocity for the ball capsule to minimize noise and wear. In this paper, we will model the ball capsule dynamics and reveal its intrinsic positive feedback structure, which is considered to be the key to realize a fast response, and design the optimal shape of the capsule to achieve the desired performance.