  We generalise the Jacob-Wick helicity formalism, which applies to sequential
decays, to effective field theories of rare decays of the type $B \to
K_{J_K}(\to K \pi) \bar{\ell}_1 \ell_2$. This is achieved by reinterpreting
local interaction vertices $\bar b \Gamma'_{\mu_1 ..\mu_n} s \bar \ell
\Gamma^{\mu_1 ..\mu_n} \ell$ as a coherent sum of $1 \to 2$ processes mediated
by particles whose spin ranges between zero and $n$. We illustrate the
framework by deriving the full angular distributions for $B \to K\bar{\ell}_1
\ell_2$ and $B \to K^*(\to K \pi) \bar{\ell}_1 \ell_2$ for the complete
dimension-six effective Hamiltonian for non-equal lepton masses. Amplitudes and
decay rates are expressed in terms of Wigner rotation matrices, leading
naturally to the method of moments in various forms. We discuss how higher-spin
operators and QED corrections alter the standard angular distribution used
throughout the literature, potentially leading to differences between the
method of moments and the likelihood fits. We propose to diagnose these effects
by assessing higher angular moments. These could be relevant in investigating
the nature of the current LHCb anomalies in $R_K = {\cal B}( B \to K
\mu^+\mu^-) /{\cal B}( B \to K e^+e^-)$ as well as angular observables in $B
\to K^* \mu^+\mu^-$.
