  Cycle switching is a particular form of transformation applied to isomorphism
classes of a Steiner triple system of a given order $v$ (an $STS(v)$), yielding
another $STS(v)$. This relationship may be represented by an undirected graph.
An $STS(v)$ admits cycles of lengths $4,6,\ldots,v-7$ and $v-3$. In the
particular case of $v=19$, it is known that the full switching graph, allowing
switching of cycles of any length, is connected. We show that if we restrict
switching to only one of the possible cycle lengths, in all cases the switching
graph is disconnected (even if we ignore those $STS(19)$s which have no cycle
of the given length). Moreover, in a number of cases we find intriguing
connected components in the switching graphs which exhibit unexpected
symmetries. Our method utilises an algorithm for determining connected
components in a very large implicitly defined graph which is more efficient
than previous approaches, avoiding the necessity of computing canonical
labellings for a large proportion of the systems.
