  It has been pointed out that the currently most precise determination of the
weak phase $\phi_2 = \alpha$ of the Cabibbo-Kobayashi-Maskawa (CKM) matrix
achieved in $B \to \rho\rho$ decays is susceptible to a small correction at a
level of $(\Gamma_\rho/m_\rho)^2$ due to an $I=1$ amplitude caused by the
$\rho$ width. Using Breit-Wigner distributions for the two pairs of pions
forming $\rho$ mesons, we study the $I=1$ contribution to $B\to \rho\rho$ decay
rates as function of the width and location of the $\rho$ band. We find that in
the absence of a particular enhancement of the $I=1$ amplitude reducing a
single band to a width $\Gamma_\rho$ at SuperKEKB leads to results which are
completely insensitive to the $\rho$ width. If the $I=1$ amplitude is
dynamically enhanced relative to the $I=0,2$ amplitude one could subject its
contribution to a "magnifying glass" measurement using two separated $\rho$
bands of width $\Gamma_\rho$. Subtraction of the $I=1$ contribution from the
measured decay rate would lead to a very precise determination of the $I=0,2$
amplitude needed for performing the isospin analysis.
