So, start selling risky assets for bonds and wait for the crash to buy back in to stocks?
People only want cash during the crash so the value of everything goes down. It doesn't matter if you bond has a known 8% yield when held to maturity; the market can't hold it to maturity so its current value drops.
Like go find 2008 in the graph of BND (Vanguard Bond ETF) vs SPY (SNP500) [1]. Let me know how you'd know when to sell your bonds for stocks.
[1]: https://www.google.com/finance/beta/quote/SPY:NYSEARCA?keymo...