An example of fibs being useful for support/resistance in historic market crashes. Dow Jones 1930's.
Shown in red and blue are the areas where a sell signal is generated when the support level is broken. To bottom when a buy signal is generated when the support holds at the bottom. Once it got through this level price did not make a new high until it's made the low of the full move. So by selling short in this area and taking profit on the second support level we’d have had a great trade. The market dropped 75% in the following three years.