What it is: A long stun.

Mechanism: Stuns your target for 2/3/4/5/6 seconds. Subject to diminishing returns.

Affected by: Improved Kidney Shot, a fifth tier Assassination talent, increases damage done to a KS target by 3/6/9%. Kidney shotting a target within 15 seconds of the last KS will decrease the duration of the KS by 50%.
Analysis: Of course, Kidney Shot shines in PvP, and is an important part of a stunlock build. The best time to KS is when your opponent has already blown his Blink/PvP trinket/etc. getting out of your Cheap Shot. But KS also has PvE uses. Dagger rogues use it for soloing and grinding, as it gives them time to run around to the back of a target and BS/Mutilate. All rogues can use it to stop a stray mob from chewing on a squishy.

And it's super helpful for decreasing damage taken by your tanks from hard-hitting trash mobs (wait until after they get enough rage!) However, most bosses are immune to stuns at all times.