A gainer is slang term for an acrobatic skill in which the acrobat performs a backwards somersault while still moving forward. In diving, this is known as reverse rotation (as opposed to front, back, or inward). Many stunt performers and martial artists are capable of performing a similar skill on the ground, sometimes in combination with a midair inverted kick.
A wrestling move called a shooting star press, innovated by Jushin Thunder Liger is very similar to a gainer, as it involves a wrestler performing a variation of a gainer from a standing position, onto an opponent (causing the wrestler to land chest first). This maneauver has many variations, ranging from different locations, different tuck styles / turns and also additional degrees of rotation, however the standard shooting star press is essentially a 3/4 gainer