The practice of swapping semen orally, that is to receive the semen from a penis, then to give it to another by kissing and allowing the semen to be swapped.
When a person receives the semen directly from the penis, then holds it in their mouth, rather than swallowing, then passes the load onto either the person who gave it to them, or to another person, by kissing.
To receive the ejaculated sperm and then to allow it to remain in the mouth. IT is then transferred to another person, either by spitting, or by allowing it to drip out, into the other person's mouth.
Popularized in Pornography, this practice is associated with gang bangs, with Bukkake style of videos.
In such videos, the object is to highlight the semen, so the recipient is usually shown with their mouth open, the assorted males then ejaculate into the open mouth, and the cum is allowed to pool and drip out.
At this point, another or several others, take turns laying below the open mouth, and allowing the semen that drips out, to drip into their own mouths.
Practice (Associated Acts): In heterosexual relations, it is considered a form of submission by the male to his female partner, where she will transfer his ejaculated sperm into his mouth.
In some instances, it is simply the desire of the male, to taste his own semen.
Noteworthy: As 'snowballing' involves the exchange of body fluids, it is not considered to be part of the 'safer sex' genre, but is considered 'high risk'.