BL Lacs for represent an extreme blazar subclass of with a relativistic jet closely aligned with our line-of-sight and characterized by non-thermal continuum emission across the entire spectrum, superluminal motion, almost featureless optical spectra and strong flux variability in all spectral bands. The latter property is crucial in discerning the innermost structure of these sources which is overwhelmed by the strong, relativistic boosted jet emission. While the fastest, subhour variability in the flaring states is widely explained in the framework of the shock-in-jet scenario (the variability triggered by the interaction of the relativistic shock front and jet turbulent inhomogeneities), those observed in lower states can be related to the instabilities in the innermost area of the accretion disc. The latter are of the particular importance: asides providing important information about the triggering instable processes, these instances also allow us to evaluate masses of the central supermassive black holes (SMBHs). In lower X-ray states of BL Lacs, such variability is more evident and easier to detect since it is not overwhelmed by the tremendous nonthermal jet emission generated near the front of the propagating shock. However, the instrumental capabilities of the current space X-ray missions are still not sufficient to reveal the subhour flux variability in lower states owing to the faintness of BL Lacs in those periods. Moreover, the planned X-ray Integral Field Unit at Athena is very important in recording high signal-to-noise 0.2-12 keV spectra within the short time intervals and study very fast spectral variability, which provides us with another powerful tool for the further progress in our understanding of the instable processes in the SMBH vicinity.