We study numerically the diffusion mechanism in silica liquid via molecular dynamics simulation. For this purpose we examine the evolution of structural units SiO(x) (x=4-6) for different times and at temperatures from 3000 to 4500 K. Simulation shows that the diffusivity of the silicon particle is performed through the transition SiO(x)→SiO(x±1), i.e., the bond-breaking and bond-reformation events. As a SiO(x)→SiO(x±1) transition occurs, one oxygen particle moves out of or into the coordination shell, leading to a collective movement of Si particles. Other types of transitions, for instance, SiO(4)→SiO(6) or SiO(6)→SiO(4), are negligible. We establish an expression for the diffusion coefficient that shows that the diffusivity is not proportional to the rate of SiO(x)→SiO(x±1) because it is strongly localized in the network structure. A high degree of localization of SiO(x)→SiO(x±1) leads to a heterogeneous dynamics. We find that the dynamics slowdown is determined by two terms: The first one concerns the change in the statistic property related to the fraction of non-four-coordinated units (SiO(3), SiO(5), SiO(6), and SiO(7)) and the second term concerns the correlation effect. Furthermore, we show that the correlation coefficient depends on both the fraction of the back-forth SiO(x)→SiO(x±1) transition and the degree of localization of SiO(x)→SiO(x±1). Our finding qualitatively supports the ideal that anomalously slow dynamics near the glass-transition point is caused by a strong localization of SiO(x)→SiO(x±1).