  We investigate to what extent the experimental information on $B \to \pi\pi$
branching fractions and CP asymmetries can be used to better understand the QCD
dynamics in these decays. For this purpose we decompose the independent isospin
amplitudes into factorizable and non-factorizable contributions. The former can
be estimated within the framework of QCD factorization for exclusive $B$
decays. The latter vanish in the heavy-quark limit, $m_b \to \infty$, and are
treated as unknown hadronic parameters. We discuss at some length in which way
the non-factorizable contributions are treated in different theoretical and
phenomenological frameworks. We point out the potential differences between the
phenomenological treatment of power-corrections in the ``BBNS approach'', and
the appearance of power -suppressed operators in soft-collinear effective
theory (SCET). On that basis we define a handful of different (but generic)
scenarios where the non-factorizable part of isospin amplitudes is parametrized
in terms of three or four unknowns, which can be constrained by data. We also
give some short discussion on the implications of our analysis for $B \to \pi
K$ decays. In particular, since non-factorizable QCD effects in $B \to \pi \pi$
may be large, we cannot exclude sizeable non-factorizable effects, which
violate $SU(3)_F$ flavour symmetry, or even isospin symmetry (via long-distance
QED effects). This may help to explain certain puzzles in connection with
isospin-violating observables in $B \to \pi K$ decays.
