Two columns show two consecutive flashes, in different wavelengths, that occurred 2.10 min apart in Spot B. Top row: WB images. The top left arrow indicates the disk-centre direction. Second row: Ca II H core images composed using the technique from Sect. 2.1. The arrow on the right points to a pair of dark structures visible in both columns. The arrow on the left points to the base of a triangular-shaped structure. Third row: same as the second row, except with the traces used to measure lengths overplotted. The contours show the umbra outlined using intensity thresholds in the WB. Bottom row: corresponding frames observed in H-alpha with white markers showing the extremities of the traces overplotted in the third row. Major tickmarks every 1′′. The Ca ii H core frames, without composition, are shown in the online movie.