I am translating a story about Till Eulenspiegel from German to English. The entire text can be found here, but I am interested in the following paragraph (from the first story about how he was baptized three times):
Und als sie über einen schmalen Brückensteg wegmussten, der keine Geländer hatte, bekam die Hebamme einen Schwindelanfall und purzelte, hast du nicht gesehen, von dem Steg in den Bach hinunter.
So I understand what is going on here - when they (a group of people) had to cross a narrow bridge without any railing, the midwife became dizzy and tumbled and fell into the stream. However, I don't understand what is the meaning of the phrase "hast du nicht gesehen" in this context and position in the sentence. I'm suspecting an idiomatic meaning as it is separated by commas (i.e., separated from the whole sentence, perhaps in meaning) and I cannot connect it to the rest of the sentence when translating it literally.