  After summarizing the current theoretical status of the four-body decay B -->
K^*(--> K pi) mu^+ mu^-, we apply the formalism of spin-symmetries to the full
angular distribution, including the S-wave part involving a broad scalar
resonance K0^*. While we recover in the P-wave sector the known relation
between the angular observables Pi('), we find in the S-wave sector two new
relations connecting the coefficients of the S-wave angular distribution and
reducing the number of independent S-wave observables from six to four.
Included in the experimental data analysis, these relations can help to reduce
the background from S-wave pollution. We further point out the discriminative
power of the maximum of the angular observable P2 as a charm-loop insensitive
probe of right-handed currents. Moreover, we show that in absence of
right-handed currents the angular observables P4' and P5' fulfill the relation
P4' = beta P5' at the position where P2 reaches its maximum.
