This is my take: lets assume a stock price today 100 points higher than it was before 100 trading days, e.g on average it went 1 point higher a day. If you had an trading system which would prefer long only, you would have a big gain, but maybe just because of beta (market performance) not your trading system.

To eliminate the long bias, you would detrend the price curve, e.g. everyday you would substract 1 point (100 point / 100 days = average per day) from the daily performance and test your system on that curve.