Joined: Aug 2008
Posts: 13,140
Likes: 2,162
From: Ann Arbor, MI
Bikes: 15 Kinesis Racelight 4S, 76 Motebecane Gran Jubilée, 17 Dedacciai Gladiatore2, 12 Breezer Venturi, 09 Dahon Mariner, 12 Mercier Nano, 95 DeKerf Team SL, 19 Tern Rally, 21 Breezer Doppler Cafe+, 19 T-Lab X3, 91 Serotta CII, 23 3T Strada
If by "spin hard" you mean putting out power at high cadence, I'd say that swaying could be indicative of an imbalance in your left/right power output/leg strength, and this would be something you'd want to work on remedying. It might also suggest your cadence is not as high as you feel it is.
If by "spin hard" you mean to grinding out power at relatively low rpm, then it's natural to have this swaying, and it's nothing to be concerned with.