Detrend "tilts" the price curve to eliminate a long-term trend so that long an short trades can achieve the same profit in average. If you use a price curve with a positive long-term trend for training, the long trades will achieve higher profits and might spoil the outcome of your training.
This is the explanation from the Black Book.

I have explained this to myself in way that testing on a long trend price curve will overweight long trades compared to short trades (and vice cersa). This is not realistic as currencys do to not have any long-term trend compared to stocks.
If the real price curve changes the trend your strategy has been trained for it might fail.

Maybe this helps...