November4: My reading is the bouncing over the torpedo nets was incidental to the reason for the bomb.
To work as Barnes Wallis calculated, the bomb had to explode at the right depth while in contact with the dam, an accuracy conventional bombing could not hope to achieve.
The backspin caused the bomb to bounce, and at the end of its run, to roll down in contact with the face of the dam, which was why it was so important for it to be released at exactly the right range and height.