SORR is the "silent killer" of retirement plans. It refers to the danger that the market performs poorly just as you begin to withdraw funds. If you withdraw 4% of your portfolio while the market is down 20%, you are effectively removing a much larger portion of your future compounding potential.
To mitigate SORR, engineers of financial systems use "Glide Paths." This involves gradually shifting from aggressive growth to conservative income as the target date approaches. By reducing volatility in the 5 years preceding and following the start of withdrawals, you protect the principal from being harvested at the bottom of a cycle.
Statistical Reality:
A portfolio that loses 20% in Year 1 of retirement and then gains 10% annually for 20 years will run out of money significantly sooner than a portfolio that gains 10% for 20 years and loses 20% in the final year—even though the average return is identical.