  Electron-phonon coupling in magic-angle twisted bilayer graphene is an
important but difficult topic. We propose a scheme to simplify and understand
this problem. Weighted by the coupling strength with the low-energy heavy
electrons ($f$ orbitals), several moir\'{e} optical phonons are singled out
which strongly couple to the flat bands. These modes have localized envelopes
in the moir\'{e} scale, while in the atomic scale they inherit the monolayer
oscillations like the Kekul\'{e} pattern. They flip the flavor of $f$ orbitals,
helping stabilize some symmetry-breaking orders. Such electron-phonon couplings
are incorporated into an effective extended Holstein model, where both phonons
and electrons are written as moir\'{e} scale basis. We hope this model will
inspire some insights guiding further studies about the superconductivity and
other correlated effects in this system.
