The Superman Hold (a.k.a. Arch Hold) is a static hold position in which the athlete starts lying face down on the floor with arms and legs extended. Arch the back such that the chest and lower thighs come off of the ground and hold that position for the prescribed duration.
An Arch Up is a variant of the Superman/Arch Hold hold in which the athlete reaches the top of the hold position, then releases back to the start position. This counts as one repetition.