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We demonstrate that the polarization fractions of most tree-dominated decays can be simply understood by means of kinematics in the heavy-quark or large-energy limit. For example, the longitudinal polarization fractions of the and modes increase as the masses of the mesons emitted from the weak vertex decrease. The subleading finite-mass or finite-energy corrections modify these simple estimates only slightly. Our predictions for the polarization fractions derived in the perturbative QCD framework, especially for governed by nonfactorizable -exchange amplitudes, can be confronted with future data. For penguin-dominated modes, such as , the polarization fractions can be understood by the annihilation effect from the operators, plus the interference with a small tree amplitude. At last, we comment …