I think I know what you're getting at. Consider this:
- - There are 10 votes. 4 for party A, 3 for party B, and 3 for null. That means neither A nor B get a majority. In many cases this would trigger a runoff.
- - The three in the previous case who voted null just stay home. 4 for party A and 3 for party B means ~57% for A, who then gets a majority.
You could imagine more values which would represent "other", "invalid choice", "explicitly undecided", etc.
To be fair, not pushing a button on that part of the machine isn't strictly equivalent to an explicit "none of the above" (more equivalent to just staying home). Perhaps leaving it blank whether you go to the polls or not ought to count as a choice - it might get some politicians to stop using low turnout as a campaign strategy.