Thanks for the explanation. I looked at the WSJ link and it seems to be saying that volatility is higher in the morning than in the afternoon. That makes sense since in the morning it is less certain how the markets will go today whereas before the closing bell we have seen the market behave for 6 hours or so.

And if you trade when volatility is high you could think it has a bigger effect on the market than if you trade when volatility is low, because in a volatile environment other traders are more likely to react abruptly to any change.

But this only works if your trades are big enough to be noticed by other traders.

It all makes sense to me now. And it makes sense that big traders who think this is the case would try to take advantage of it (I don't think it can be made illegal, or is it already?).

Therefore we can assume that prices are artificially high in the morning, and so it makes sense for us small guys to sell in the morning, buy before the close -- if you are a small investor. The big guys will of course do the opposite, to accomplish their market-manipulation.