What Are Bone Conduction Technology and Bone Conduction Glasses?
Bone Conduction is basically a nonstandard way of sound transmission. All the old-school headphones/earphones use the air as a medium for sound transmission which means that sound travels from the source (headphone) through the air and through your outer, middle, and inner ear, to your eardrums and then to your cochlea. Instead of using air, bone conduction headphones use bones for sound transmission. That way, the sound bypasses your outer and middle ear (and eardrums) and gets transmitted directly to your inner ear.
This simple difference between the old-school and bone conduction headphones causes all the differences in design and working principle and, furthermore, it makes them suitable for different things.
Instead of standard mini loudspeakers you can find in regular headphones and earphones, bone conduction glasses/headphones feature small vibrating speakers/transducers that go onto your cheekbones. They produce the sound which can be heard but they also vibrate and those vibrations are transmitted through your bones. This small difference in working principle is the thing responsible for all the advantages and disadvantages of bone conduction sunglasses/headphones.