Using high‐speed photography to investigate the transient response of bubbles to a bias field pulse, a shift in the geometrical center has been related to the previous propagation history. For left (right) skew bubbles, a low‐mobility section appeared at the left (right) side of the previous propagation direction. The location of the hard wall section in respect to the propagation direction agrees with Thiele’s vertical Bloch line theory. Applying this method to low‐angle propagators, i.e., bubbles with few Bloch lines, an asymmetric expansion was found after propagation in a gradient drive field that was higher than a certain threshold. For these bubbles, hard wall sections of oppositely wound vertical Bloch lines pinned the two flanks of the bubble and reduced the mobility of the rear allowing the front to expand out asymmetrically. These vertical Bloch line bundles with opposite sense were generated in the front and rear, then clustered on the two flanks during the gradient propagation, consistent with a dynamic converion model.