Essentially, they tricked voters into leaving the 'booth' after pressing the "Vote" button on the ES&S iVotronic. That button, does not actually cast the vote, as one might think (and as these voters were told), but instead, it brings up a review screen of the voter's "ballot."
Instructing the voters that they were done, the conspirators then, after the voter had left, would change the voters' votes as they saw fit, before finally pressing the "Cast Ballot" button.
It's not "voter fraud" it's "election fraud" we need to watch for.
