  Recently we have shown that due account of Wigner spin rotations is needed to
match the ISGW model consistently onto HQET. We now discuss the flavour
dependence of this procedure. We find that for finite mass ``heavy'' quark the
universal function $\xi(y=1) \approx 1$ but the slope is proportional to the
energy of the spectator quarks, and ratios of form factors are corrected in a
way that QCD sum rules seem to require. Predictions for heavy quarks, such as
in $B_c \rightarrow \psi(\eta_c)$ and to mixed systems, such as $D\rightarrow
K$ and $B \rightarrow \pi$ show a systematic mass dependence that may be
confronted with data.
