He tried to explain it to me, but I had trouble understanding it, too. As I remember it was a telescoping-type box , rather than a flap-style , and he rigged it to cut open the bag when the lid was removed. It's been 40 years, so the memory is about as fuzzy as Bigfoot's behind."and then rig the box with a razor blade that would cut the bag open when the box was opened."
For the life of me I can't envision the mechanism that would do this.