Ly$alpha$ absorption spectra of QSOs at redshifts $zsimeq6$ show complete Gunn-Peterson absorption troughs (dark gaps) separated by tiny leaks. The dark gaps are from the intergalactic medium (IGM) where the density of neutral hydrogen are high enoug
h to produce almost saturated absorptions, however, where the transmitted leaks come from is still unclear so far. We demonstrate that leaking can originate from the lowest density voids in the IGM as well as the ionized patches around ionizing sources using semi-analytical simulations. If leaks were produced in lowest density voids, the IGM might already be highly ionized, and the ionizing background should be almost uniform; in contrast, if leaks come from ionized patches, the neutral fraction of IGM would be still high, and the ionizing background is significantly inhomogeneous. Therefore, the origin of leaking is crucial to determining the epoch of inhomogeneous-to-uniform transition of the the ionizing photon background. We show that the origin could be studied with the statistical features of leaks. Actually, Ly$alpha$ leaks can be well defined and described by the equivalent width $W$ and the full width of half area $W_{rm H}$, both of which are less contaminated by instrumental resolution and noise. It is found that the distribution of $W$ and $W_{rm H}$ of Ly$alpha$ leaks are sensitive to the modeling of the ionizing background. We consider four representative reionization models. It is concluded that the leak statistics provides an effective tool to probe the evolutionary history of reionization at $zsimeq5-6.5$. Similar statistics would also be applicable to the reionization of He II at $z simeq 3$(Abridged)
