a dive in which the diver takes off facing forward and performs a backward half-somersault, entering the water headfirst and facing the springboard.
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Examples from the Web for half gainer
But the sound of his phone ringing startled him so much he nearly did a half-gainer off the roof.Makers|Cory Doctorow
British Dictionary definitions for half gainer
a type of dive in which the diver completes a half backward somersault to enter the water headfirst facing the diving boardCompare gainer
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012