Activision Teenage Mutant Ninja Turtles - Danger of the Ooze - Manual (Page 6)

Shell KickerS
At the end of an attack combo, the Turtles will start to spin their
weapons while they wait for a finishing move – these finishing
moves are called Shell Kickers!
To perform a Shell Kicker, hold F while the Turtle is spinning
his weapon (keep in mind the number of attacks in a combo
is different for each Turtle). While the Turtle is spinning his
weapon, move left stick Up, Down, Forward, or Back – it’s that
easy!