What is a sprag unit and what does it do?

A sprag is a kind of clutch that will turn in one direction but not in the other.

There are many designs but the common one is a set of balls on ramps. When going the wrong way the rollers run up the ramp assisted by the spring and jam rotation. When it's rotated the correct way the rollers are constantly being forced down allowing rotation.

The most common place in a transmission to use this is the stator in the torque converter. The stator hold still starting from a stop to redirect fluid and will spin when at cruise speed.

This kind of clutch has many applications. The bendix of a starter contains one and will spin freely for a few seconds when the engine start but the starter is still engaged. Some alternators have one built into the pulley to reduce the spinning momentum of the engine when the RPM drop suddenly. Hellicoptors have it for if the engine fails the blades can still rotate.

There are several versions of sprags. Some use rollers as mentioned. Others use oblonged feet or wedges.