Rhythm Explosion [Source] is the Glee Club run by Sandy Ryerson from 1997 until 2009, only seen in the Un-Cut Pilot episode. Sandy was then fired for inappropriately touching Hank Saunders, a student in the club.
In the Uncut Pilot, they perform One from Broadway musical A Chorus Line. Will remarks at how they were a stool choir and the worst in the McKinley High School Glee Club world. Will also mentions that every member has a MySpace page due to feeling invisible. It is also shown that Tina, Rachel, Mercedes, and Kurt were members of the club, with them being the only holdovers to continue with New Directions.