  Mean field dynamo theory is a leading candidate to explain the observed large
scale magnetic fields of galaxies and stars. However, controversy arises over
the extent of premature quenching by the backreaction of the growing field. We
distinguish between rapid mean field dynamo action, which is required by
astrophysical systems, and resistively limited action. We show how the flow of
magnetic helicity is important for rapid action. Existing numerical and
analytic work suggesting that mean field dynamos are prematurely quenched and
resistively limited include approximations or boundary conditions which
suppress the magnetic helicity flow from the outset. Thus they do not
unambiguously reveal whether real astrophysical mean field dynamos are
dynamically suppressed when the helicity flow is allowed. An outflow of
helicity also implies an outflow of magnetic energy and so active coronae or
winds should accompany mean field dynamos. Open boundaries alone may not be
sufficient for rapid dynamo action and the additional physics of buoyancy and
outflows may be required. Possible simulation approaches to test some of the
principles are briefly discussed. Some limitations of the ``Zeldovich
relation'' are also addressed.
