The hair-cells along the Basilar membrane respond selectively to specific frequencies.
The physical wrenching of the hair-cells, and the corresponding generation of an electrical potential is the actual instance of transduction.
Not all of the hair cells, however, are stimulated by a sound wave. Two factors account for this. One is that the Basilar membrane is actually thicker (more mass) towards the back of the room (top of the snail shell). Hence, it resonates more for low frequency vibrations.
The Base of the Cochlea resonates more for high frequency sounds. The other is that low frequency sound waves tend to be longer and more penetrating, whereas high frequency waves are short.
The result is that the hair-cells along the Basilar membrane perform much like the keys to a piano. That is, hair-cells at specific locations respond to specific frequencies.