Stationary and nonstationary multilevel models are formulated to analyze ultrafast multiphoton processes with emphasis on electronic and nuclear coherence of a near-resonant intermediate state. By identifying and characterizing three intrapulse dynamical mechanisms (sequential-resonance, chirp-following, and time-delay), a clear physical picture is developed for understanding the subtle chirp dependence in intrapulse pump-probe processes. The theoretical analysis in the paper provides not only a possible interpretation for a recent three-photon experiment but also a general theoretical framework for studying ultrafast multiphoton processes in atoms and molecules.