the Dow Quotes offered by the SB site tend to follow the futures price +/- a fair value, based upon time to expiry, Alan's Greenspan's Body Mass Index, and the number of candles in China.
seriously though, not just outside the NYSE opening hours, but also during them, the SB price of the cash index follows the futures +/- this adjustment, which is why the SB price doesnt always tally with the cash price which your charting system may be telling you.
even before the futures market opens, then the SB companies also manipulate the dow price to tally with the Asian/European markets, and to some extent the FX market.
The SB generally takes a view on market direction.
In fact, I used this as a tactical play when I used to use SBs.
If you can get a look at 2 or more SBs on the same instrument you can see where their bias is in relation to the underlying, i.e. toward the top or the bottom on the range if they had a view and middling on the rare occasion when they didn't have the faintest.
Then, when you begin to see them move away with no real movement in the underlying, you get a pretty good indication which way they think it's going to go.
Difficult to give empirical example as it's more of a 'feel'.
Say the underlying (whatever instrument your following) is making it's move - you don't care which direction or if it's just ranging.
You're also shadowing (more than one) SB to see what they're doing with their price.
Although they will very roughly be following the underlying you will notice, if they have a view, they will be 'hugging' one side of the range. So, for instance, they'll be quick to move their price with the index in one direction, but a little slower in the other. You can pick up their bias from this.
Very occasionally you'll get a bonus and see them switch emphatically from either one end to the other or more often, from a middle (haven't got a view) position to emphasis.
Probably not made it much clearer, but I think you can get the gist.
You can watch futures on the Dow on www.igindex.co.uk home page or any of the other spread bet home pages. You can see after hours movements on the Dow, in fact I have put bets on/closed bets out on the Dow around 23:00!
Also watch the Dow before and after 07:00..can change a lot at that time.