Simply put closing the window allows the FMC to get back to VNAV PTH, above it, it will go IDLE/HOLD - VNAV SPD below it will go to THR/HOLD - VNAV SPD the 1250' may only be in the ground school computer based training.
Once in HOLD I often add thrust to reduce the descent rate even further and allow the FMC to catch the path as early as possible.
The 1250 is only a initial attempt I believe based on the FMC performance figures for current weight etc. once in HOLD it may end up more or less however it'll always be better than an idle descent.