Generally, the automatic gearbox uses a roller clutch for regime upshifts/downshifts. It operates identical to a ratchet. It transmits the torque only in one direction by free-wheeling or overrunning in the other. The advantage of such a clutch is the fact it eradicates the possibility of concurrently implementing the clutch https://damienftbin.blogocial.com/the-best-side-of-gearbox-auto-69562755