My gf sucks my nuts pretty hard and will pull them a few inches before letting them 'pop' back into position. Pretty nasty and always allows my to cum more after repeated sucking and popping. She'll also squeeze both nuts in her hand when she blows me or jerks me off. To be honest, I'm surprised how much sustained squeezing of my nuts I can take. My nuts will ache slightly after a heavy sex session...it's sort of enjoyable pain I guess!