"You say that CMC lays off on futures for any net buying/selling they receive at any one time, say, from their Dow index trading customers. Is that right?"
I'm not sure I totally understand your question as I didnt refer directly to CMC : however, all the SB companies "cash" index prices are driven by the underlying futures market. This goes some way to explaining the SB bias, because the futures market is more volatile and moves quicker. Hence, when the futures start to tank/rocket, the SB price moves BEFORE the cash chart price moves. Some of this can be worked around because certain SB companies (CMC is one, I think) drive thier own charting package off their prices, rather than a third party for price information.Hence, thier charts accurately reflect the SB price movement.
Have a look at the attached chart, this might make it a bit simpler : the first chart shows Dow futures at 2.30BST on Friday. The second example shows Dow cash at the same time. The two large white bars are the cash market rising rapidly at open in order to "catch up" with the futures market. This price difference will already be reflected in the brokers price and thier quotes will open at the higher level. However, the chart feed (driven in my example by MyTrack) would be behind the SB price. The SB broker, of course, knows where the futures are before the market opens, so his quotes do not allow this rapid climb to be exploited, otherwise we'd all be having the SB companies pants down every day. This isnt really a fair example but it does show the difference clearly. Normally there is only a few seconds (sometimes less) between the futures chart and the cash chart.
Also note how much more volatile the futures are. These are both 1 minute charts, yet the futures chart shows more spikes on the candlesticks than the cash chart, which looks quite smooth. Also note the difference in the MA lines ; my 100EMA line (the blue one) on the futures chart is pretty close to the price, because the futures normally trade quite flat prior to the open (news excepted), yet on the cash chart is moves rapidly up.
Hope this helps.
Matt