  We investigate the $P$-wave states $T^-_{bb}$ in the isospin singlet and
three excited modes [excitation occurring in the diquark $[bb]^{s_1}_{c_1}$
($\rho $-mode), antidiquark $[\bar{u}\bar{d}]^{s_2}_{c_2}$ ($r$-mode) or
between them ($\lambda$-mode)] from diquarks in a quark model. We analyze the
dynamical behaviors of the diquark $[bb]^{s_1}_{c_1}$, antidiquark
$[\bar{u}\bar{d}]^{s_2}_{c_2}$ and their correlations in the states $T^-_{bb}$
by decomposing the interactions from various sources in the model. The absolute
dominant color-spin configuration, more than $99\%$, in the $\rho$-mode with
$1^1P_1$ is $[bb]^0_{\bar{\mathbf{3}}}[\bar{u}\bar{d}]^0_{\mathbf{3}}$. Its
energy is lower by about $18$ MeV than the threshold $\bar{B}\bar{B}$ so that
it can establish a compact bound state. The chromomagnetic and meson-exchange
interactions in the antidiquark $[\bar{u}\bar{d}]^0_{\mathbf{3}}$ are
responsible for its binding mechanism. Two other excited modes are higher than
their respective threshold. The color configuration
$\mathbf{6}\otimes\bar{\mathbf{6}}$ need to be handled discreetly in the
tetraquark states.
