Contrary to popular female belief, Blue balls is a very real condition that results from a prolonged state of sexual arousal. The term blue balls relates to the appearance of a bluish tinge due to loss of oxygen in the blood that's been held in the scrotum for a while. I recently had a case of the blue balls so severe James Cameron called me several times in regard to filming for Avatar 2.
To quickly go over the mechanics of it, when a guy is physically turned on, blood flows to his penis causing an erection, as well as increased blood flow to the testicles causing them to swell. If he doesn't ejaculate, there is a buildup of pressure and the balls unfortunately feel the brunt of it. With sensations ranging from a mild ache to a full on nut cracker where it feels you've been kicked in the junk.
Luckily the condition isn't dangerous, BUT this is a closely guarded secret we don't want the women folk to find out about. Its best they continue to go on thinking that they are saving our lives by giving out those helping "hands" (and blow jobs), a valued service to the male community. where was I, anyway the release of "tension" (thank you lady's) causes the blood to drain, and any discomfort to subside and all is once again good in the world (in my pants).