  A toy model for gamma-ray burst supernovae (GRB-SNe) is discussed by
considering the effects of screw instability of magnetic field in black hole
(BH) magnetosphere. The screw instability in the Blandford-Znajek (BZ) process
(henceforth SIBZ) can coexist with the screw instability in the magnetic
coupling (MC) process (henceforth SIMC). It turns out that both SIBZ and SIMC
occur inevitably, provided that the following parameters are greater than some
critical values, i.e., (i) the BH spin, (ii) the power-law index describing the
magnetic field at the disk, and (iii) the vertical height of the astrophysical
load above the equatorial plane of the rotating BH. The features of several
GRBs are well fitted. In our model the durations of the long GRBs depend on the
evolve time of the half-opening angle. A small fraction of energy is extracted
from the BH via the BZ process to power a GRB, while a large fraction of energy
is extracted from the BH via the MC process to power an associated supernova.
In addition, the variability time scales of tens of msec in the light curves of
the GRBs are fitted by two successive flares due to SIBZ.
