Oppositely directed magnetic field lines converge at a current sheet, reconnect, and release stored magnetic energy as heat and particle acceleration.
Sweet-Parker reconnection rate: v_in/v_A = 1/√(L·v_A/η) = S^{-1/2}, where S = Lundquist number. Petschek reconnection (η-independent) operates through slow shocks. Solar flares release ~10³² erg via reconnection of kilogauss coronal fields.