Depending on configuration of VNAV it may assume a 1000'/min descent rate until reaching the platform = new CRZ Altitude.
Be careful with this as the T/D generated is for new CRZ Altitude, thus if not careful you may get high before flying the approach if your aircraft has not yet reached this new altitude.
Therefore I would strongly recommend you not to use this primative method.
However if you have an extended level segment (such as an early descent profile London ATC can give you [descent from CRZ @ FL390 to fly 80nm @ FL250]) then it would be prudent to enter this new level as a new cruise level so ECON CRZ speeds for actual Cost Index , Gross Weight, Wind, OAT and FL can be calculated and flown, rather than the ECON fixed descent speed which is only based on CI.