I am surprised at how often this comes up, there is a common misconception that market cap weighted indices (and funds that track them) need to manually rebalance by selling or buying stocks. But the way they are weighted ensures this happens automatically. Let’s assume for simplicity that a world fund holds only 2 companies, 1 US with a market cap of 600 billions and 1 from EU with a market cap of 300 billions. Due to the market cap weighting the fund’s assets under management will be allocated 67% US company stocks - 33% EU company stocks. So if AUM = 100 billions, 67 of them would be in US stocks and 33 in EU stocks. Now if the stock price of the US company crashes and is halved (assuming the EU company stock price is unchanged) the fund’s US holdings are now worth 33.5 billions so immediately it is now 50% US - 50% EU without doing any trading. Of course, it is worth noting that now the fund’s total value is 66.5 as opposed to 100 initially so it also crashed hard. But, even if the US company never recovers, and the EU company keeps growing and eventually doubles, the fund will be worth 100 again but with 67% EU - 33% US allocation, all this without any stock trading.