Yoko Geri Keage, or side snap kick.
According to Bruce Clayton, the only sensible use for it is a snap kick to the groin when an opponent opens themselves up with a high kick of their own. I can also see it used as an in-close finishing kick if the opponent is down or on the way down, or as a vital point kick if you can land it somewhere like an underarm or throat.
Research has shown me that the kick was never an original kick in Karate, but came about in the 1930’s when tournaments disallowed kicks below the waist. Historically the side kick we now use in Kanku Dai, etc., was originally a front kick which can be seen if we study the Goju Ryu karate style.
As Dr. Clayton says, the Yoko Geri Keage is a Shotoism, then.
Karate, like life, isn’t about right or wrong – it’s about doing what you do well.