The Buoy is designed to be off-balance, like its namesake bobbing device, though not as extremely as a Pilates ball; the idea is that the microadjustments you're continually making with your body are not annoying enough to be a hassle, but adequate to burn some calories. Also unlike a Pilates ball, the Buoy is height-adjustable.
"I like to think of you in halves," I said. "Half warm and welcoming and comforting..." I reached towards his one blue eye as my voice lowered to a whisper. "Half stern and unfeeling and absolutely chilling to the bone."