Transmit electrical signal across human body
Why does it have to use the body?  Bluetooth LE works just fine from one  side of the body to the other, even at minimum power, say -12 to -18dBm.  I'm guessing it mainly uses reflection off whatever is nearby, it would get attenuated stongly on the direct path. 

Now, to try to design what you're talking about: assume the two devices each touch the body with some conducive part. At audio to low mhz the body is a largeish capacitor (roughly nF), but you need to complete a circuit around that somehow.  Assume the devices each have a metal case isolated from the body acting as antenna.  The cases are small capactors to free space in the pF range, assuming a device which is at least a few inches in size (parasitics, basically).  This gives you an ac circuit, but the free space side of it is very weakly connected.  Also you are limited to very, very small currents through the body: I believe 50 microamp is one safety standard.  If you can design a circuit that sends data through two wires with 1pf and 1nf capacitors in series on them while limited to 50uA, you can use that :) because that is equivalent to what you have through and around the body.

The described scenario is related to having ordinary rf antennas in each device: part of the signal is still transmitted as an electromagnetic field around the body, but instead of using dipole to dipole coupling (in approximately the far field),  this can be monopole to monopole (and near field).

EDIT Also there are parasitic capacitances body to free space  and each case to body, which form a capacitive divider. The result is maybe a picofarad across the two wires in the equivalent circuit described above.