Spoiler: the undocumented WZ registers are just microcode temporaries to hold large operands. For instance when reading a jump instruction, the jump destination can't be read directly into PC since that would mess up reading the second half of it, so it's read into WZ and then WZ is transferred to PC. This is invisible to the programmer.

One might say the instruction is really "jump to WZ", or JWZ, which, of course, refers to Jamie Zawinski (not really).