It is observed that a large heat flow abruptly transfers across the x point (which can be located at both the high field side and the low field side) of the 1/1 or 2/2 magnetic islands during a sawtooth crash on the HT-7 tokamak by using tomography of the high-resolution soft-x-ray emission together with the singular value decomposition technique. Although the poloidally asymmetric heat flow contributes an m = 1 component of perturbation in the signals, the heat flow is obviously different from the growth of the m = 1 magnetic island. Furthermore, it is also shown that the large heat flow or the displacement of the hot core occurs only after the crash. Hence, the large displacement of the hot core is just a result of the crash rather than the cause. The heat flow from both the x points of the m = 2 island is also observed. This observation demonstrates that a purely fast reconnection of the m = 1 magnetic island is not responsible for the crash, but that it may be due to the rapid energy and particle diffusion in the stochastic region.