  Single pulse behaviour of radio pulsars is usually interpreted in terms of
the ExB drift of a radio beam. It is shown that antisymmetric arrangement of
the radio-bright zones can produce several types of observed single pulse
phenomena: the half-cycle jump in subpulse modulation phase, left-right-middle
subpulse sequence and switching between the core-dominated and cone-dominated
pulsation modes. The geometry can also produce nulling, both sporadic and
intermodal. The model implies that the radio-quiet intervals that separate the
main pulse and interpulse in PSR B0826-34 correspond to azimuthal breaks in the
radio beam, instead of breaks in colatitude.
