A moment of unlimited potential, where energy and creativity are released in high yields. (A more descriptive term for "being in the zone", the phrase it replaces.) The conditions necessary for a creative reaction are a well-engineered infrastructure, simplified business procedures, and productivity skills which enable creatives to shut out distractions, make the most of available time, and be energized. When these conditions are met, the spark of creativity is all that is needed for a creative reaction to occur.
I shut my door, turned the ringer off, then quit out of chat & e-mail. I got SO MUCH of my screenplay done. Creative reaction is the only way to describe it!