Remember your level changes are mostly achieved through the legs but initiated by the trunk (midsection). I'll call it dipping rather than ducking so as to get away from thinking bending at the waist. When you're slipping a punch most of that movement is achieved through "shoulder rolling". When in a fight and during dipping or slipping punches or dealing with other strikes you keep your eyes on their center-mass and basically unfocus your eyes in a way to let your peripheral vision detect strikes/movement from any part of their body. That and the hands "go" with the head - don't let them drop during head movement.