  We develop a model for subpulse separation period, $P_2$, taking into account
both the apparent motion of the visible point as a function of pulsar phase,
$\psi$, and the possibility of abrupt jumps between different rotation states
in non-corotating pulsar magnetospheres. We identify three frequencies: (i) the
spin frequency of the star, (ii) the drift frequency of the magnetospheric
plasma in the source region, and (iii) the angular frequency of the visible
point around its trajectory. We show how the last of these, which is neglected
in traditional models by implicitly assuming the line of sight through the
center of the star, affects the interpretation of $P_2$. We attribute the
subpulse structure to emission from $m$ anti-nodes distributed uniformly in
azimuthal angle about the magnetic axis. We show that variations of $P_2$ as a
function of rotational phase or observing frequency arise naturally when the
motion of the visible point is taken into account. We discuss possible
application of our model in signifying overall field-line distortion at the
emitting region. Abrupt changes in $P_2$ can occur during state switching in
the magnetosphere. We demonstrate that the unique value of $P_2$ in each
rotation state can be used, in principle, to relate the rotation state of the
magnetospheres to subpulse drifting.
