The opening of the fingers and more importantly the extension of the wrist adds tension to the forearm which facilitates a quicker or should I say more beneficial use of the pull over of the off arm. An arm with an extended wrist "wants" to be pulled across the body and will be more connected with the torso.
Try it yourself and you will notice the difference between extended and non extended
Yonex VCore 100s - SW 351 6pts HL
Tour Bite / Luxilon NG 16 @ 51lbs