In a crash, you spend down your bonds/cash avoiding the sale of shares whenever possible until they again reach a reasonable price. Expect large dividend cuts to follow a crash/recession by a couple quarters.
Most big crashes recover in 2-3 years. If you had a $1M stache, $40k/year in nonnegotiable spending, and a 12% bond allocation, you could spend your bonds and exit a 3 year dip with the exact same number of shares you started with.
Actually you come out better than that. If your original $880k in equities throws off 1% in dividends (that's factoring in a huge dividend cut, index-wide) you exit those difficult 3 years having sold only about 94k of your bonds.
So on the imaginary day in 3 years when everything returns to their pre-crash levels at the same time, your shares are again worth $880k and your remaining bonds would be about $26k. Your now-$906k stache took a hit, but your retirement is secure. Yes, your WR is now 4.4% instead of 4% and your allocation is 3% bonds instead of 12% bonds, but your shares are restoring their dividends and growing faster in an economic environment where the underbrush has burned off. Your survival at this WR and allocation is still a very high probability, and chances are good you'll see $1M again in a year or two. Crashes tend to be followed by several years of great returns, so it's a good thing you held onto your shares!