The exact process of how "new" light is generated has never been explained because, if it were, even more questions would arise. I expect they toss around different ideas, play with the wording, pray (what exactly happens when they do?), go away, mull it over, meet, keep playing with the wording, invite their writers to write something about it. But, in essence, apart from fine-tuning the basics, there's not much for them to do. It's a well-oiled machine that grinds on and on. All they need is nice new ways of saying what they already teach. Kind of like a 9 to 5 spiritual job. Occasionally, a snag arises such as the end not coming within expected time frames, so cautious reframing and re-interpretation's required. Not a biggie at all; hence they pull "over-lapping generations" out of the bag, and, wow! The rank and file are so impressed and satisfied!
All very prosaic and ordinary. As with most religious authority, once it's in place, it becomes its own justification. People in authority can write and speak with power - and that power can appear to followers to be divinely sanctioned.
The human tendency to submit to authority is scary but understandable. The alternative is to be virtually "at sea" with those stock and trade questions beloved of religionists who subscribe to "modern" Christian beliefs. Silly, unknowable questions like, Why Does God Permit Wickedness? or What is the meaning of life (answer: To submit to our directions on what you should do with your life).