Rebalancing is a thing, though generally for risk reasons. It would/could have saved one's returns during the so-called Lost Decade of the 2000s with the S&P 500:
As your equities dropped, there's a good chance bonds would have at least stayed neutral, or even risen: so you'd sell some of those (sell high) and pick up equities at a discount (buy low).
There are even products available that do this automatically for you: