  The cumulative light curves of gamma-ray bursts (GRBs) smooth the spiky
nature of the running light curve. The cumulative count increases in an
approximately linear way with time t for most bursts. In 19 out of 398 GRBs
with T90 > 2s, the cumulative light curve was found to increase with time as
\~t^2 implying a linear increase in the running light curve. The non-linear
sections last for a substantial fraction of the GRB duration, have a large
proportion of the cumulative count and many resolved pulses that usually end
with the highest pulse in the burst. The reverse behaviour was found in 11 GRBs
where the running light curve decreased with time and some bursts are good
mirror images of the increases. These GRBs are among the spectrally hardest
bursts observed by BATSE. The most likely interpretation is that these effects
are signatures of black holes that are either being spun up or down in the
accretion process. In the spin up case, the increasing Kerr parameter of the
black hole allows additional rotational and accretion energy to become
available for extraction. The process is reversed if the black hole is spun
down by magnetic field torques. The luminosity changes in GRBs are consistent
with the predictions of the BZ process and neutrino annihilation and thus
provide the link to spinning black holes. GRBs provide a new window for
studying the general relativistic effects of Kerr black holes.
