Not a 767 driver but from several jumpseat rides on the 757/767 this is how I understand it to work:
For climb:
VNAV will use climb thrust and pitch to hold the ECON climb speed (based on cost index?) to the lower of: entered cruise altitude, MCP altitude. It will respect any speed restriction (like 250Kts/FL100) and this supersedes ECON speed.
For descent:
The FMC will calculate the T/D (Top of Descent) point based on the first altitude restriction on the way down. So if you have entered to be level at DAYNE @ FL70, it will calculate an idle descent profile from the cruise altitude to reach FL70 at that point. This is the most economic descent available.
In the descent itself, VNAV will command rate of descent down the calculated descent profile and apply thrust as required to maintain ECON speed (again with any entered restriction being respected).
Sorry to any 767 drivers if I have made a mistake here, this is just what I remember to be the case. Please feel free to correct / elaborate on my attempt!