As you probably know, the phrase freak dancing is used as an unoffensive way to say "fuck dancing" but means the same thing. The girl grinds her genitals, in time to the
music, gently against the
boy's clothed genitals, with the aim of producing a climax of either or both dance partners. Girls have been wearing mini-skirts without panties in order to make this dance movement even more enjoyable for themselves.
Dipping is a further development of freak dancing. The girl unzips the
boy to enable a
dip, usually one brief shallow penetration in time to the
music.
It is not considered sex, which is far more prolonged. It is more like giving each other a brief "kiss" with the genitals. If a
boy tries to prolong a
dip, he is avoided for at least the rest of the evening by all the girls at the dance.