They don't guarantee you zero downside compared to investing in the index, though, but compared to putting your money under the mattress.
It's relatively easy to achieve a return profile like these promise with some combination of Treasuries and index options (at least while Treasuries pay 5%!), and the ETFs are doing this kind of financial engineering rather than promising to beat the market through stock-picking skill.
It's relatively easy to achieve a return profile like these promise with some combination of Treasuries and index options (at least while Treasuries pay 5%!), and the ETFs are doing this kind of financial engineering rather than promising to beat the market through stock-picking skill.