Quote from Don Bright:
Our traders make extra $$ with price improvement a couple of ways. As you say, the (more often than not) retracement is always great. But the pairs traders get an extra boost. If, for example, they have an offer to sell short at .10, leaning on the other stocks offer at .18 for example (meaning that they want to pay 8 cents for the spread, and they get price improvement on the sell side, and still get the buy side done, they may buy the 8 cent spread for 3 cents (bigger numbers in reality of course).
We do the same thing on Nasdaq, we do rest orders at specific prices, and when hit take the other side (or at least evaluate the spread at that time, based on market factors, etc.).
Don