Blowing air is when a man has cum so many times in such a short space of time that he's got nothing left in the locker to ejaculate but air. It hurts, but the end result can be spectacularly mind numbing in its intensity.
A male experiencing the contractions of orgasmic release without ejaculating is steeped in 5000 years of ancient Chinese folklore. According to eastern practitioners, male orgasm with ejaculation is one fleeting moment of intense and even excruciating pleasure. On the other hand, the ability to orgasm without ejaculation is said to be a continual rolling expansion of the orgasm, with emensley heightened ecstasy.
Unfortunately, to learn this art, a man would need to spend years at an eastern ashram, performing spiritual and physical exercises designed to develop the skill. The reason this art is so rare in western cultures is that most modern men see something patently wrong with some old, foul-smelling, skinny, toothless, bearded Swami freak wearing nothing but a turban and a loin cloth touching their perineum point for demonstration purposes.
Warning: any user of the term should be aware that this phrase may be perceived in some circles to be crude or vulgar. In these situations, saying 'passing air' would be more politically correct.