A lot of things set a market trend but the most important thing is the herd mentality of the people doing the actual buying and selling. It doesn't matter why the market trends it matters that you stay in gear with the trend.
Find a good site on the net (cbsmarketwatch is one) and look at the chart of the S&P vs the 200 day simple moving average for the last decade. Just imagine if you were smart enough to sell when the market went under the 200 day and bought it back when the market went above it as in: sell in 2000 and buy it back in April of 2003. Now those were some trending trades.
You answered my question with an equities example, I specifically asked about currency. I understand backward looks at charts to see a trend. I want to know why you think you know what drives or trends the currency market, or the bond market (if you want to go for that one).
Oh. and for what it's worth, the wise man was the chief currency dealer at a large German bank.
And my personal favorite, "Emotion is a contrarian indicator."