This is a very interesting trend following strategy. It sounds similar to the turtle trading strategy Richard Dennis came up with. In theory, I just read the book “The Complete Turtle Trader” by Michael W. Covel because its on Pipcrawler’s list (Pipcrawler’s Favorite Trading Books). Essentially, the traders would buy what was going up instead of waiting for a retrace which people tend to do and miss the trend. You might get some ideas from the book or better understand why it works. Donchian’s 4-week rule and Gann’s “Truth of the Stock Tape” have also been helpful to me to understand trend following.
I think more important than whether or not it should or shouldn’t work is that it does seem to work for you, so stick with it!
One last thing, it does seem that some forex pairs are positively and negatively correlated. This page might be worth looking into Forex Market Pair Correlations | Best Currency Pairs to Trade. You might increase your winnings by not buying pairs with negative correlation.