For a fraction of a second, the performer inhales strongly, pulling mucus from the outer part of the nasal cavity higher up, even into the sinus. This action is generally repeated every few seconds or minutes.
Reducing the need to sniffle generally involves reducing the symptoms causing the excessive mucus, often through antihistamines or decongestants. More immediate relief is had by either nasal irrigation or blowing the nose into a facial tissue or handkerchief.
Many people are unconscious of their sniffling, hence the stereotype of children as more apt to sniffle, since they are less conscious of stigma. The sharp high pitched noise of a sniffle can easily become both inaudible to the performer, who experiences it as a relief, and irritating to those around the sniffler.
In many cultures, blowing one's nose in public is considered impolite, and in reaction, people can make a habit of sniffling, even though sniffling is more impolite.