Thanks again for clearing this up.
And to confirm this in writing, these are excerpts from the Big Boeing Book:
Selection During Descent:
If Speed Intervention is selected during descent,
while on, above, or below path, in speed reversion,
or on a VNAV PTH segment, pitch control immediately
becomes VNAV SPD and the A/T mode becomes
HOLD.
Transition from Speed Intervention:
When the MCP speed knob is pushed again to
deselect speed intervention, the target speed becomes
the VNAV DES page target speed and path
capture is enabled.
The THR mode is selected if the
airplane is below the path to set the thrust for
closure onto the path. HOLD is selected when the
thrust is set to allow the flight crew to adjust the
closure rate onto the path.
Resumption of the descent when the airplane is below the
descent path results in descent guidance of VNAV
SPD and an initial thrust setting (THR) to achieve a
flight path angle that is half the angle for the
descent path. A/T HOLD occurs after the initial
thrust target is reached.