HTupolev

With a typical unclutched rear derailleur, the pulley cage uses a simple sprung pivot to tension the chain. This is fine for smooth roads and stuff, but when you're riding on chunky surfaces or doing gnarly stuff, the chain can get bouncy. It is somewhat possible to counteract this by using a stronger spring in the derailleur's cage pivot, but this causes the lower run of the chain to be under more tension all the time, which has some undesirable effects: the derailleur won't shift as readily and lightly, and it increases drivetrain friction.

A clutched derailleur addresses this by modifying the mechanism used for the pivot. The spring still exists, but the pivot also includes a friction plate, similar to the mechanism used in a friction shifter. When the chain tries to bounce and force the derailleur's cage to swing forward and release chain, the friction plate resists that motion. But, there's an obvious problem with this: we don't want the friction plate to impede the derailleur's ability to wrap chain back up. So the friction plate is attached to a one-way clutch that causes it to "disengage" when the cage is wrapping up chain (rotating clockwise when viewed from the drive side). So the "clutched" derailleur behaves responsively like a traditional derailleur when wrapping up chain, but adds resistance against motions that let chain out chain, and it does all this without necessarily requiring constantly higher chain tension than a traditional derailleur.

There are other ways to address the problem. For example, the new SRAM AXS rear derailleurs use a fluid damper on the pivot. This damper is not directional like a clutched friction plate: instead, it works by impeding fast motion in general. The fluid damper applies very little resistance to slow movements, like what the cage experiences from gear shifting. But when there are fast jerky movements, the fluid applies a LOT of resistance. It's like aerodynamic drag: it's not linearly proportional with your cycling speed, it ramps up with increasing steepness as you go faster.