"Ball," or testicle, sucking can cover a number of mouth-to-scrotum activities (the scrotum being the sack of skin containing the testes). Except for the biting, chewing, and swallowing, it could be likened to eating a lollipop: a sucker might put her/his mouth around one or both of a partner's balls, licking, tugging, and even nibbling in a way that's pleasurable to all involved. Because the genitalia are full of sensitive nerves and tissue, testicle tasting and other oral sex entrees need to be done with care and clear communication.
Sucking and tongue action can be combined with hot breaths, as well as blowing cool streams of air, which can provide additional sensation and drying. Alternating from one ball to the other can also suffice as some men may feel more sensation in one testicle versus the other. It's also common for one testicle to hang down further than the other, so this, too, might dictate which side gets sucked. And because temperature affects how high or low the "fruit" will hang, testicle-sucking sessions may vary in look and feel.
Shifting the focus for a minute to the sucker, s/he might have a one or two ball preference. This could be determined by the pleasure, or lack thereof, felt from a "full mouth," and/or the need for more breathing room. As with other sexual play involving the mouth, stray pubic hair can also be on the menu; for those who find this natural occurrence distasteful, a quick rinse and spit with water should do the trick. Some men and women shave some or all of their pubic hair to prevent hair loss during sex, among other things. http://goaskalice.columbia.edu/all-about-ball-sucking