Yeah, that comment confused me at first - but I got his point:
O'Donnell's supporters are more motivated, and they'll vote no matter what. The smaller overall turnout would mean that a greater percentage of the voters would be from this 'motivated group'. If there was a large overall turnout, the 'motivated group' would be watered down by the shiftless sorts who had nothing better to do than to go to the voter's booth today -- and might vote for Castle because they were familiar with his name...
At least that's what I think that was his theory.
It doesn't matter - just vote and don't let the media, or other hecklers, discourage you. Man proposes but God disposes...
Great advice: The “El Cid Rule.”