  Our experiments show that in $^4$He crystals, the binding of $^3$He
impurities to dislocations does not necessarily imply their pinning. Indeed, in
these crystals, there are two different regimes of the motion of dislocations
when impurities bind to them. At lowdriving strain $\epsilon$ and frequency
$\omega$, where the dislocation speed is less than a critical value (45
$\mu$m/s), dislocations and impurities apparently move together. Impurities
really pin the dislocations only at higher values of $\omega$. The critical
speed separating the two regimes is two orders of magnitude smaller than the
average speed of free $^3$He impurities in the bulk crystal lattice.We obtained
this result by studying the dissipation of dislocation motion as a function of
the frequency and amplitude of a driving strain applied to a crystal at low
temperature. Our results solve an apparent contradiction between some
experiments, which showed a frequency-dependent transition temperature from a
soft to a stiff state, and other experiments or models where this temperature
was assumed to be independent of frequency. The impurity pinning mechanism for
dislocations appears to be more complicated than previously assumed.
