The pups do not pick up vibrations through the body. They only register disturbances in their electro-magnetic field. Such as the strings vibrating or the phone in your pocket connecting to a network.
Other than that, good question, fun to read up on!
Let's get started with a bit from Wikipedia:
" In music, the undertone series or subharmonic series is a sequence of notes that results from inverting the intervals of the overtone series. While overtones naturally occur with the physical production of music on instruments, undertones must be produced in unusual ways. While the overtone series is based upon arithmetic multiplication of frequencies, resulting in a harmonic series, the undertone series is based on arithmetic division."