looks like he's set up so he has to arch his back in order to relieve the tension on his balls. maybe before the third pic was taken the 
 cord hadn't been ratcheted up so high? as far as ways to make it more intense... maybe include an element of cock worship? you could stand in front of his face and demand that he praise/beg for your cock, and anytime he get's to quiet (being distracted with keeping his back arched) you could ratchet the cord up a little higher.