Hi Honk,
Matchspeed is pretty wide of the mark regarding the Des Now function.. 'Des Now' is an option to start an early descent. In all instances when you're in Descent Mode but under the glide path, VNAV will command a 1250ft/min descent in the hope of capturing the idle path. It'll do this in IDLE/HOLD or THR, along with VNAV SPD. When within 150ft of the idle path, it'll transition to IDLE/VNAV PTH.
To answer your question - you obviously know that VNAV goes into point-to-point descent path construction, whether that is from the first to subsequent constraints, or from your present position to the next constraint (ie. If you recycle the next constraint waypoint in the legs page).
So, how to create an idle descent path in VNAV after the. First constraint, rather than the point-to-point profiles it will draw, there are 2 options.
The first, and the one I prefer to use; is...
As you approach the first waypoint constraint on descent, let's say it's MUSEL FL130; once you're assured you'll make the constraint, but before it Has captured the ALT mode (IDLE/HOLD will change to SPD) - delete the restriction in the legs page. Or, if the MCP alt is set to a lower level than FL130, just push the button.
VNAV will then re-calculate an idle descent based on the next constraint, you've removed the first one before you've 'obeyed' it.
The second, is simply to re-cruise the FMC at FL130 once you're there, as Machspeed said. That is, set 130 in the VNAV CRZ page, as the cruise altitude. Again, it'll plan an idle descent based on the next constraint.
Hope this is of some use.
With all that being said, in the environs we operate in these days, idle descents are basically just a wish, so I generally don't bother doing either; just stick to the 3 times tables, and when made to descend below the path, do it in V/S500 or VNAV SPD, and nudge the thrust levers up a bit when in HOLD, to achieve a shallow descent.
Cheers.