Estimate to Complete Time Phasing

In working with cost-loaded schedules in P6 v7, I was asked how P6 spread the Estimate to Complete.  I realized I had no idea.  In particular, does P6 use the assigned resource curve?  One of the issues we've run into is a steep jump in the forecast spending for work that is running under budget but is still forecast to finish on or near the budgeted value.  P6 appears to assume that the spending will return to normal in a single period.  Graphing this produces as an unrealistic jump in the ETC.  So the question is, is there a way