We predict a new physical mechanism to explain the electron spin precession frequency focusing effect recently observed in singly charged quantum dots exposed to a periodic train of resonant circularly polarized short optical pulses [ A. Greilich et al. Science 317 1896 (2007)]. We show that electron spin precession in an external magnetic field and a field of nuclei creates a Knight field oscillating at the frequency of the nuclear spin resonance. This field drives the projection of the nuclear spin onto the magnetic field to the value that makes the electron spin precession frequency a multiple of the train cyclic repetition frequency, the condition at which the Knight field vanishes.