This is from the wikipedia article:
“Seven participants made out their ballots using the opti-scan paper sheets (Hursti remaining outside the test area). Sancho then went to Hursti and gave him a ballot which Hursti filled out. Hursti then gave Sancho the memory card to insert in to the machine. The operation of the machine was explained by Sancho to those in attendance and the card inserted and machine turned on which then produced the “zero total tape.” The tape produced zero votes cast. The test ballots were then inserted in to the Diebold machine followed by the “ender card” (same size as ballot) was inserted telling the machine to turn off its counting function and start its reporting function. The machine then produced a paper tape with 7 yes votes and 1 no vote.” (from Wikipedia)
I was curious about the back story and he pointed me to this paper which goes into extreme detail of how he discovered the weaknesses and how he was able to demonstrate them. There are many other descriptions of what he did, but this paper, in his own words, is great because of the dry scientific way in which he describes what was done.