  We consider the dynamics of the domain-wall kink soliton, in particular we
study the zero mode of translation. In the infinitely-thin kink limit, we show
that the zero mode is almost completely frozen out, the only remnant being a
dynamically constrained four-dimensional mode of a single but arbitrary
frequency. In relation to this result, we show that the usual mode expansion
for dealing with zero modes -- implicit collective coordinates -- is not in
fact a completely general expansion, and that one must use instead a
traditional generalised Fourier analysis.
