Quote from CFerret:
Hmm... I will probably appear a complete amateur, but it all sounds like a kind of Mumbo-jumbo for me. 
I mean how can this knowledge be aplied to real trading?
How does it not apply? New traders seem to think buy low, sell high is all there is to trading but there's so much more. It's about buying at the lowest risk price and selling at the highest return price (or vice versa for shorts).
A couple examples for NYSE:
-You are a momentum breakout trader. You are building a position in XYZ as it starts looking ready to run. Now put in bids also down 10c, 20c, and 30c in case of a large sell print. If you get a fill then you have a great average that allows you a wider stop, and higher profit if the stock really does break out. It also allows you to add more on the breakout at higher prices because you have the wider stop. This just gives you that little edge over the other breakout traders.
-You are a pairs trader, and you have a core position that you trade around in a pair. If you envelope both stocks all day you can occasionally get a great print to get a better average on your pair, again allowing you wider stops and more profit when the pair contracts. A little edge over the other pair traders.
Both of those examples use price improvement which other markets don't offer. I made living off the specialist handling rules for the last 4 years. The recent NY changes have made the strategy not worth it for me anymore, although still profitable.
A dated Nasdaq example from 1999-2001 was when there were many ECN's (8 or 9) and day traders all had access to different ones depending on the broker. Not many people had access to INCA which often meant you could pick them off for 20-30c after the market has moved. An example of an innefficiency that is now pretty much gone.
These are the kinds of methods that people use to make a consistent living in trading. It all applies!