"Attain stillness while moving..."
Ah, Bruce, what sweet words you use (Bruce Lee, not Springsteen).
Anyway, this is a statement from Lee's writings, specifically a chapter on combat mobility. He goes on to use the analogy of the moon's reflection in water, still and quiet despite the rolling and breaking waves. It's good, sound, Taoist stuff, which Lee used in many of his martial analyses. He broke down technique endlessly, all the while acknowledging the crucial--yet intangible--mental elements of training that serve to elevate the skill of a martial practionioner.
I think stillness while moving transcends combat.
One who is aware of his/her surroundings while walking along a busy downtown sidewalk is Still While Moving. One who considers what repercussions his/her actions will have amongst others is Still While Moving. One who stays true to his/her honour while out working and exploring in the world is Still While Moving. And one who trains in a combative art for the sake of self-betterment and with non-aggressive goals is Still While Moving.
The moon in the river is Mind. It is the steadfastness of Spirit despite the ever-changing backdrop of the world we live in.