Their boosts are reversed.
Well at least Showdown has it like that. :P. Did a quick Shuckle + Malamar test. Shuckle boosted with Contrary Shell Smash to +2 Def, SpD and -4 Atk, SpA, Spe. Topsy-Turvy reversed it to -2 Def, SpD and +4 Atk, SpA, Spe. No replays since the replay thing is broken.