  Though the small-scale, low-ionization knots, filaments and jets (LISs) of
planetary nebulae (PNe) are known for ~30yr, some of their observational
properties are not well established. In consequence our ability to include them
in the wider context of the formation and evolution of PNe is directly
affected. Why most structures have lower densities than the PN shells hosting
them? Is their intense emission in low-ionization lines the key to their main
excitation mechanism? Therefore, if considered altogether, can LISs line
ratios, chemical abundances and kinematics enlighten the interplay between the
different excitation and formation processes? Here we present a spectroscopic
analysis of five PNe that possess LISs confirming that all nebular components
have comparable electron temperatures, whereas the electron density is
systematically lower in LISs than in the surrounding nebula. Chemical
abundances of LISs versus other PN components do not show significant
differences as well. By using diagnostic diagrams from shock models, we
demonstrate that LISs' main excitation is due to shocks, whereas the other
components are mainly photo-ionized. We also propose new diagnostic diagrams
involving a few emission lines ([NII], [OIII], [SII]) and
$\rm{log}$(f$_{shocks}$/f$_{\star}$), where f$_{shocks}$ and f$_{\star}$ are
the ionization photon fluxes due to the shocks and the central star ionizing
continuum, respectively. A robust relation differentiating the structures is
found, with the shock-excited clearly having
$\rm{log}$(f$_{shocks}$/f$_{\star}$)$>$-1; while the photo-ionized show
$\rm{log}$(f$_{shocks}$/f$_{\star}$)$<$-2. A transition zone, with
-2$<\rm{log}$(f$_{shocks}$/f$_{\star}$)$<$-1 where both mechanisms are equally
important, is also defined.
