JTAG is basically a giant shift register with a "bit" at each pin that overrides its normal state. You can shift in H, L, or float for each pin, and then shift out the sensed voltage.
Most of the chips I've seen that have JTAG, have a section detailing how it works for that chip, since at least the pin sequencing needs to be documented. Google for "jtag tutorial" for more.
