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.
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