Conjecture: For an eruptive flare/CME launch to occur, some subset of the coronal magnetic field lines must shorten during the process of energy release.
Comments: There are no rigid walls in the corona, so the field must find its own volume. On the time scale of a flare or CME launch, the photospheric boundary condition (vector field) must remain fixed (eg Melrose, 1998).
Proof: Level surfaces of the volume integral of B2 must decrease to diminish the total magnetic energy... remainder of proof TBD!
Implications: The source of a "magnetic explosion" must be an implosion in at least a part of the coronal volume. There is not much evidence for large-scale inflow, and on the other hand the most intense flare emissions are normally compact structures. Ergo it seems likely that the magnetic implosion that drives a flare and/or CME eruption must originate in strong small-scale fields.