The **reverse drum with low one-way clutch** in an automatic transmission:
1. **Enables Smooth Reverse Engagement** – Prevents harsh gear lockup during shift to reverse.
2. **One-Way Locking** – Freewheels in forward motion but locks in reverse (prevents mechanical conflict).
3. **Supports Planetary Gearset** – Works with sun/ring gears to reverse output direction.
4. **Reduces Shock** – Absorbs driveline torque reversals.
Failure causes reverse slip or forward/reverse clash!