What does the 20 day EMA signify?
The 20 EMA often acts as support and resistance and is watched by a lot of traders, many of whom look to short rallies to the 20 EMA during downtrends, and buy dips to the 20 EMA during uptrends.
It probably becomes something of a self-fulfilling prophesy in much the same way as Fibonacci retracements do.
There are quite a few trading system that are built around using the 20 EMA as a reference point.
Mind you, there are still plenty of good trades from retracements that go nowhere near the 20 EMA.
The trick with any retracement is to trade them in the direction of the trend so that your profit potential is magnified.