DOPMA - Defense Officer Personnel Management Act is a law that manages Officer promotions. It mandates that joint experience be given promotion preference, the up or out system, career milestones, and several other areas that affect our day to day life.
The cynic in me knows that all the talk about pilot shortage, dual tracks, less staff positions, etc etc will not change until Congress modifies/repeals/adjusts DOPMA. Sec Gates through Sec Carter have tried making adjustments, but get constant kick back from the Pentagon. Personal opinion, it is one of the few ways to "fire" officers and is the system that promoted the General Officers we have today.
DOPMA does mandate that if continuation is declined, an officer will be separated NLT 6 months from notice of non selection. Our AFIs match this and say a DOS can be moved up earlier as well.
It's kind of a back door way out of an ADSC early, at the cost of closing some doors. To go to the USAFR or ANG, it takes a bit more leg work (waiver) but is doable (pendulum swings.) Someone who has gone through the process can probably speak more intelligently to it.
https://en.wikipedia.org/wiki/Defense_Officer_Personnel_Management_Act
https://dopma-ropma.rand.org/index.html
Hope this helps!