Based on weak field quantum control theory, a semiclassical approximation relates the characteristic parameters of the tailored Light field to corresponding classical dynamical quantities and thus reveals the underlying physical basis of wave packet focusing. A coordinate-dependent two-level-system approximation is employed to further analyze the molecular dynamics induced by short laser pulses, thus leading to a simple interpretation of the observed correlation between the pulse chirp and vibrational focusing and defocusing. Though our study is presented in the context of quantum control, the conclusions are general, providing an intuitive picture of the quantum coherence of light-matter interaction and a guideline for the design of tailored laser fields.