What is a compounding wing and how does it increase helicopter performance?
This paper discusses wing compounding and appears to be a free PDF download.  Apparently the added wing provides lift to offload the rotor, allowing them to work more on providing forward propulsion and relieving some of their need to also create lift.  Figure at the top of page 6 shows the helicopter with added wing with outboard props on the wing.


  The compound helicopter configuration, known as the hybrid compound helicopter, features both wing and thrust compounding. The wing offloads the main rotor at high speeds whereas two propellers provide additional axial thrust as well as yaw control. This study investigates the maneuverability of these two helicopter configurations using inverse simulation. The results predict that a hybrid compound helicopter configuration is capable of attaining greater load factors than its conventional counterpart, when flying a Pullup-Pushover maneuver. In terms of the Accel-Decel maneuver, the two helicopter configurations are capable of completing the maneuver in comparable time-scales. However, the addition of thrust compounding to the compound helicopter design reduces the pitch attitude required throughout the acceleration stage of the maneuver.