  A new drifting pulsar, PSR J0815+09, was discovered in the Arecibo drift-scan
searches. An intriguing feature of this source is that within the four pulse
components in the integrated pulse profile, the sub-pulse drifting direction in
the two leading components is opposite from that in the two trailing
components. In view that the leading theoretical model for pulsar sub-pulse
drifting can only interpret one-direction sub-pulse drifting, the observed
"bi-drifting" phenomenon from PSR J0815+09 poses a great challenge to the
pulsar theory. The inner annular gap (IAG), a new type of inner particle
accelerator, was recently proposed to explain both $\gamma$-ray and radio
emission from pulsars. Here we show that the coexistence of the IAG and the
conventional inner core gap (ICG) offers a natural interpretation to the
bi-drifting phenomenon. In particular, the peculiar drifting behavior in PSR
J0815+09 can be reproduced within the inverse Compton scattering (ICS) model
for pulsar radio emission.
