  We evaluate the W-exchange diagram within the framework of light cone sum
rules (LCSR), taking $B_d^0 \to D_s^-K^+$ as an example. This decay mode,
though proceeding via the W-exchange diagram only and therefore expected to be
highly suppressed, has the branching ratio $3.2 \times 10^{-5}$. We estimate
the W-exchange amplitude within LCSR, including soft gluon corrections, to
twist-3 accuracy. The calculation naturally brings out the features which
suggest that such an amplitude is expected to be small in cases when both the
final state mesons are light, without relying on any kind of an assumption.
With minor changes, it is also possible to have a rough estimate of the
exchange/annihilation type contributions to other processes like $B \to \pi
\pi,~\pi K, KK$. We find that though it appears as if the sum rule method
yields a fair agreement with the observed value, a careful analysis of
individual terms shows that the method, in its present form, is inadequate to
capture the correct physical answer for the case of heavy final states.
