The formation of hard bubbles by single-pulse bias field in epitaxial garnet films was investigated in some detail. It was verified by experiments and calculations that, in general, VBL in hard bubble walls are not generated by the way that one Bloch line pair is produced per pulse. The dependence of the hardening of soft domain segments on their movements was revealed by means of the method of double exposure photography. Two kinds of favorite movements for hard bubble formation were discovered and the physical meaning of the" demarcation field for soft and hard bubble formation" H[b] was elucidated.