Furbies have a single reversible motor, which drives a series of gears and cams which moves his ears, eyes, eyelids, mouth and butt. The cams are all on the same drive shaft, so one full rotation of the shaft will put him through all combinations of motions. His patterns of eye, mouth, ears and butt motions are predetermined by the patterns on the cams. He cannot independently move his eyes from his mouth, for example. However, he does have a separate cam for each major function, so by careful arrangement of the lobes and shapes of the cams, moving a body part separately is possible in a limited fashion by moving the main shaft back and forth in a narrow rotation of the cams.
Each angular region of the shaft's rotation can be described as combinations of each motion controlled part's state (ie: eyes open, eyes closed, ears up, ears down, mouth open, mouth closed, etc.) By arranging these states on the cams, sequences of animated motion can be achieved by rotating the main shaft forwards and backwards. For example, to dance, he may drive the main shaft back and forth 20 degrees, moving the butt motion cam, but that happens to be a dead spot for the eyeball and ear cams, making his eyes stay open and ears unmoving during the dance. However, the positions cannot be accessed randomly, only sequentially (ie: in order to get to "dance" he may need to spin through "eat", "blink" and "sleep" to get there).
All of the moving body parts are clutched, spring loaded, or otherwise protected from being obstructed or manipulated by careless humans trying to restrict Furby's motion.. Unfortunately, the gears and cams are made from a softer, pliable plastic. This may protect them from damage by allowing them to flex under stress, but it also allows them to get into invalid positions under stress and jam up easily. Some of the guides and gear parts are also very thin and easy to break. Of course, a furby is a $30 toy, so certainly no one should expect a rare alloy steel inside of them.
Furbies' literature claims they have three different voice "pitches". If these are represented by different voice ROM masks, then I would guess the smaller daughter PCB is the voice ROM and the larger PCB would be the CPU and software. However, it may be that the "pitches" are simply minor variations in the sound playback rates and not represented as ROM changes at all.