The Milankovitch insolation equation as an astronomical forcing function for climate change is modified to include the frequency variation effect of the Earth's obliquity on insolation. To establish the criterion that frequency variation of the obliquity is responsible for major paleoclimatic changes, the cooperative phenomena between the obliquity frequency and the Milankovitch insolation are defined as frequency-insolation resonances. Coupling of the orbital frequency and insolation variations has established an especially new and interesting series of these resonances, which may induce pulsation of insolation. In this paper we show that insolation pulses would modulate the Milankovitch insolation in such a way that the mode (phase or period) of the insolation variations could be locked into a 100-kyr cycle. The pulse-modulated Milankovitch insolation forcing is tested by energy balance climate models. The physical mechanism behind this forcing for glacial-interglacial changes is explained and clarified in terms of pulse duration in the incoming solar radiation. It is concluded that the frequency-insolation resonances may be the key to the puzzle of the 100-kyr paleoclimate cycle.