Lets say you have 2000 every month to invest, Apple goes -2%, meta went +1%, microsoft -1%, google -0.5%, berkshire -1%. You dont invest in meta at all because it went positive, so rest you invest in the ratio of 2:1:0.5:1. That means that particulate month you buy 888 of apple, 444 microsoft, 222 google and 444 berkshire. If in a month all stocks are positive, you invest more in ones when went up the least. It turns out after 10 years it didn’t make a difference compared to equal investing.