The OP is referring to a testing idea on page 172 of LeBeau and Lucas's
(book) on futures trading.
L&L wanted to measure the relative "goodness" of entry techniques
independent of the exit used. (They also wanted to measure the relative "goodness" of exit techniques, independent of the entry used. The Big Idea was, of course: find the best entry, find the best exit, put em together, Bam. A continuous firehose blast of profits into your account. Bada bing bada boom.)
L&L settled upon a simple test for entries: after entering, hold the trade for a fixed number of bars, N, (you get to choose N), and then exit on the close. Count up the percentage of these "trades" that were profitable, neglecting commission and slippage. The entry positioned you in the right direction to profit from the N-day move. Higher winning percentage is better.
Their idea is, if you were an idiot coin-flipper, randomly entering at a random time in a random direction (long or short), and then holding for N bars, you ought to have a winning percentage of about 50%. Since you want your entry method to be "better than random", you want to see a winning percentage ON THIS TEST higher than 50%. L&L picked an arbitrary measuring stick, 55%, which certainly is higher than 50%. But so is 54% and so is 56% and so are many other arbitrary numbers. In real trading, some trades would be stopped out waaay before the Nth bar and so in real life, the winning percentage will be different than what this test indicates.
I think a fair criticism of this procedure is: it's old. It solves a problem that just doesn't exist any more. LeBeau and Lucas's book is almost 20 years old; since it was published, system testing software has gotten tremendously better. Now, unlike then, you can test zillions of entry methods WITH zillions of exit methods. You don't have to "divide and conquer" the problem by choosing an entry independently of choosing an exit. Computer hardware and software has advanced quite a bit since then.