A gainer is a backflip performed moving forwards. It is usually either a tuck or flashkick. The former is usually performed off of an object, whereas the latter is usually done with a J-step
Even though it consists of a backwards rotating mid-air somersault , it is not actually a backflip variation . A gainer has it’s own variations and is an entirely different flip.
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Gainer Tutorial - 3RUN
Tucked gainer tutorial