When you extend your elbow so that it resembles the upper fin of a dolphin, or shark and jam it into your victims back, or scrotum
. Mostly done by bullys and neighborhood thugs.
Bully: " I gave this ladythug
a dorsal fin and her spine almost cracked."
Thug: " Oh yeah, well I dorsal finned this guys scrotum so hard that his testes fell out."