We consider the dynamical pair-creation by intense electric fields in vacuum, based on exact solutions of the Dirac equation using the temporal Klein process. This approach gives a simple interpretation of the QED processes associated with arbitrary time-varying electric fields, and leads to the standard quantum kinetic equation. As an application, we consider the case of an intense laser field containing a large spectrum of coherent high-harmonics (HH). The role of the fast field oscillations is discussed. We show that HH pulses eventually lead to an enhancement of pair-creation and can be described as a sequence of dynamically equivalent Sauter pulses.