I've seen three claims (four if you count this thread.)
One: Undecided break for the challenger.
Two: Undecided break for the incumbent.
Three: Undecided break proportionally to the decided.
Four: (On this thread) Undecided don't vote.
Five: (My observations) Undecided break for the cutest canditate.
Six: (My observations) Undecided break for the taller candidate.
Seven: (My observations) Undecided break for the candidate listed first on the ballot.
If you are really undecided...don't you think you are most likely going to stay home? When the majority of people do stay home...the undecideds by and large are the non-voters who get polled. It makes sense really. A lot of people are going to answer a poll saying they are registered and vote etc...makes them look like a better citizen to the pollster. In actuality they may have no intention of voting and say undecided to the pollster because they don't have a clue about those running and undecided is a safe response.