  We performed numerical simulations to study the response of
magnetohydrodynamics (MHD) to large-scale stochastic forcing mechanisms
parametrized by one parameter, $0 \le a \le1$, going from direct injection on
the velocity field ($a = 1$) to stirring acts on the magnetic field only ($a =
0$). We study the multi-scale properties of the energy transfer, by splitting
the total flux in channels mediated by (i) the kinetic non-linear advection,
(ii) the Lorentz force, (iii) the magnetic advection and (iv) magnetic
stretching term. We further decompose the fluxes in two sub-channels given by
heterochiral and homochiral components in order to distinguish forward, inverse
and flux-loop cascades. We show that there exists a quasi-singular role of the
magnetic forcing mechanism for $a \sim 1$: a small injection on the magnetic
field $a < 1$ can strongly deplete the mean flux of kinetic energy transfer
throughout the kinetic non-linear advection channel. We also show that this
negligible mean flux is the result of a flux-loop balance between heterochiral
(direct) and homochiral (inverse) transfers. Conversely, both homochiral and
heterochiral channels transfer energy forward for the other three channels.
Cross exchange between velocity and the magnetic field is reversed around $a =
0.4$ and except when $a \sim 1$ we always observe that heterochiral mixed
velocity-magnetic energy triads tend to move energy from magnetic to velocity
fields. Our study is an attempt to further characterize the multi-scale nature
of MHD dynamics, by disentangling different properties of the total energy
transfer mechanisms, which can be useful for improving sub-grid-modelling.
