I understand the 'W' button disengages the locking ring on the torque
converter. This normally engages after a gearchange so the change is
smooth, but the mechanical efficiency after is as good as a manual clutch.
Without it the converter is only 80-85% efficient & torque transfer would be
limited, hence it is best permanently disengaged when high torque to the
wheels is not needed.
As far as I'm aware there is no limitation on the gear selection when this
is in use in Drive mode. The older style of auto which has manual 1-2-3
selection is intended for towing or driving up/down hills, where unplanned
upshifts could be dangerous.
