I was thinking about a similar idea to this yesterday. Something like holding 60/40 or 70/30 and then reallocating to 90/10 or 100 in the event of a crash.
I'm reading so much material so quickly that it's all starting to mash together, so I can't cite the source correctly. Either Swedroe or Bernstein explored this strategy. There are some nuances/caveats to it.
First, if you do it, make sure your bonds are treasuries or the highest quality bonds. Don't chase yields while you're waiting for the market to crash. The reason is that junk bonds actually track with the market during major crashes. In other words, they'll be down at the same time the market is, so the assets may not be there to shift into stocks. In 2009, every asset class was down except treasuries, CDs, and cash.
Second, this is essentially market timing, which is contrary to a passive indexing strategy. You can't predict the market. What happens if the bull market keeps running for another year, or two, or three? Will you be sitting with lower returns until then?
Third, this requires great courage to actually execute. Have you invested during a major market crash? In 2008, banks were failing, investment companies were failing, insurance companies were failing, local governments were running out of money. The major car companies almost went out of business. The US government was nationalizing private companies (they didn't call it that, but that is essentially what you call it when the government owns GM.) Major firms (Lehman Brothers, Merrill Lynch) that had been around for 100 years closed. Everything feels like a watershed moment has occurred, that things are different this time. And that's when you're proposing to drop all bonds and go 100% all in. It's easy to plan for it, but hard to actually execute in the moment. In addition, you may not buy in at the bottom. If you buy in, and the market slides another 30%, then stays there for five years, will you be able to stay the course? Ask a Russian, they're going through this right now. The currency is collapsed, they're under sanctions, oil prices (that they depend on for revenues) are low and staying there -- is this the best time to buy into the Russian stock market?
The conclusion was that this is actually a very aggressive strategy. Sitting on the sidelines during a long bull market, then buying in during a bear is actually hard to do. Great returns require great risk. Great risk means you can lose your money.
I'm thinking of a less aggressive variation. Following bond recommendations to temper risk allows participation in the stock market, but lowering the bond allocation a little during market downturns may enhance returns. I'm still learning and thinking about it, though.