Like the pronator teres, joining the lower and outer part of the humerus (note 1) to the radius (note 2), this muscle rotates the forearm, this time from a palm-down to a palm-up position ("supination"). Though not carrying a large load, it may be important in archery as it is one of the the muscles holding the string hand vertical (ie keeping the palm of the string hand parallel to the string).
On this diagram, the muscle overlays the more important finger flexors.