  Both nulling and subpulse drifting are poorly understood phenomena. We probe
their mechanisms by investigating how they interact in PSR B0809+74. We find
that the subpulse drift is not aliased but directly reflects the actual motion
of the subbeams. The carousel-rotation time must then be over 200 seconds,
which is much longer than theoretically predicted.
  The drift pattern after nulls differs from the normal one, and using the
absence of aliasing we determine the underlying changes in the subbeam-carousel
geometry. We show that after nulls, the subbeam carousel is smaller, suggesting
that we look deeper in the pulsar magnetosphere than we do normally. The many
striking similarities with emission at higher frequencies, thought to be
emitted lower too, confirm this. The emission-height change as well as the
striking increase in carousel-rotation time can be explained by a post-null
decrease in the polar gap height. This offers a glimpse of the circumstances
needed to make the pulsar turn off so dramatically.
