The mental model for this state is known as "flow" or atheletes sometimes call it "the zone"
So the productivity multiplier advice is this: learn to cope and be productive at doing the kinds of work where flow is impossible/unfeasible!
(This way you'll not be already tired and behind when you get to the work for which "flow" is possible...)
Flow happens during fun.
Sometimes there are shortcuts to grinding that can only be discovered during "flow" (usually for me, it's to identify better tools)