This Quidditch manoeuvre is used by the Keeper to protect as large an area as possible. To accomplish this move, the Keeper hangs by one hand and one foot from his or her broom, extending the other hand and foot as far out as possible (QA10).
Starfish are marine invertebrates, usually with five arms around a central disc. The position of the Keeper during this move, resembles this if you count his/her head as the fifth arm. (https://en.wikipedia.org/wiki/Starfish)