Final answer:
The anconeus muscle in the posterior compartment performs a unique function of assisting in elbow extension, unlike other muscles which primarily focus on wrist and finger extension.
Step-by-step explanation:
Within the posterior forearm compartment, most muscles are primarily involved in extensor functions. However, when considering the function that is unlike all others in this group, the anconeous muscle stands out. It is unique as it assists in elbow extension and is not primarily a wrist or finger extensor. This muscle helps in controlling the ulnar abduction during forearm pronation and is not solely dedicated to actions at the wrist and fingers like the other posterior muscles such as the extensor radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, and the extensor carpi ulnaris, which are all involved in extending the wrist and fingers.