Trending is any series of higher highs and higher lows, or of lower highs and lower lows — at least two of each. Two methods for applying Fibonacci retracements with a trend can either signal when the trend is being reversed, or when a reversal attempt is failing.
The first use is to measure the difference between a trend’s countertrending. The 61.8% difference between the two prior lows/highs is significant support/resistance. At least a reaction can be expected, regardless of its ultimate resolution.
The second use is to measure an attempt to reverse a prior reversal that had failed. Reacting back into the trend’s last countertrend reaction tends to retrace it 61.8% — and then usually resume the trending. A more significant reaction can start to signal the trend is reversing.