What is the difference between No Action, e.g. the Monk's Centered Flurry of Blows [ddi], and the more common Free Action?
There are conditions that prevent characters from taking actions altogether (stunned, unconscious, dying, etc). This prevents free actions, but a "No Action" power could still be used. Also note that most powers that enhance your initiative are No Action because you are using them before combat technically begins.
My guess at the reason this particular power is a No Action power is that you can normally only make one attack as a free action per turn. "No Action" powers don't count toward this limit.
Free actions take almost no time or effort. You can take as many free actions as you want during your or another combatant’s turn. There is an exception to that rule: A creature can take a free action to use an attack power only once per turn. Creatures don’t normally have attack powers that can be used as free actions, but some powers and other effects grant the ability to use an attack power (usually a basic attack) as a free action. For example, a character might have two different abilities that let him or her make a melee basic attack as a free action when their respective triggers occur. If both abilities are triggered on the same turn, the character can make only one of the melee basic attacks during that turn.
Centered Flurry of Blows could be argued to be an attack power, and thus use up your 1 free action attack per turn. Giving it a "No Action" ensures that it doesn't.