I don't understand them. Take the Mav's/Rockets game that is on right now. The Mav's are up 17, and the halftime line is +6 for them. Shouldn't it be the other way around?
Typically, if one team wins the first half by a decent margin, the other team wins the second half. This is because they have less of a reason to be aggressive and they can sit back a bit, playing a more defensive game and protect their lead. Also, certain teams tend to score more points in either the first or second half. For example, when on the road, Cleveland scores much more in the first half than the second, so if you see them up by a lot in the 1H, they're more prone to giving up the 2H.
Thanks for the info. Kind of obvious now that I think about it. I took both the Mav's and Cleveland for the second half though so I hope they keep it up.