Planetary nebulae (PNe) are known to possess a variety of small-scale structures that are usually in a lower ionization state than the main body of the nebulae. The morphological and kinematic properties of these low-ionization structures (LISs) vary from type to type in the sense that LISs can appear in the form of pairs of knots, filaments, jets, and isolated features moving with velocities that either do not differ substantially from that of the ambient nebula, or instead move supersonically through the environment. The high-velocity jets and pairs of knots, also known as FLIERs, are likely to be shock-excited. So far, most of the FLIERs analyzed with ground-based small and medium telescopes, as well as with the HST, do not show the expected shock-excited features -either the bow-shock geometry or the shock excited emission lines. In this talk we discuss the crucial problem of the excitation mechanisms of FLIERs -through the comparison of jets and knots of NGC 7009 and K 4-47- and what might be the contribution of large telescopes.