You may have missed something important, which is you should have some ideal portfolio balance in mind when purchasing. This forces you to “buy low and sell high” in that you buy whatever you need to to bring your percentages in line with you’re ideal. This is the only strategy I’ve seen that provably beats the market (and it’s been a while since I saw the paper talking about this, may be that it only beats the market in terms of total risk adjusted returns, and not total dollars returned).

Edit: also it’s important that you’re asset portfolio has asset classes that tend to be out of phase, like stocks vs bonds.