The Relative Strength Index

Core Mechanics and the Rubber Band Model

1. The Rubber Band Mental Model

The Metaphor: Imagine a stock price attached to a rubber band. As the price moves rapidly in one direction, the rubber band stretches, and the RSI measures that exact tension.

Interpreting Tension: When the RSI hits 70 or 80, the rubber band is stretched to its limit; while it does not mean the price must fall immediately, it indicates that the energy to stretch further is running out, making a snap-back toward the centre increasingly likely.

2. Calculation and Threshold Boundaries

Standard Formula: The indicator is calculated using average gains and losses over a specified lookback period (typically 14 days) via the formula: RSI = 100 - [100 / (1 + RS)] where RS represents the average of X-day up closes divided by the average of X-day down closes.

Clinical Limits: Fixed boundaries at 70 (traditionally marking overbought conditions) and 30 (marking oversold conditions) serve as baseline guides for extreme price exhaustion.

Advanced Interpretation: Divergences and Failure Swings

1. Spotting Divergences

Bullish Divergence: Occurs when the price of a security prints a new low while the RSI forms a higher low, signalling that underlying selling pressure is drying up.

Bearish Divergence: Forms when the price touches a new high while the RSI makes a lower high, warning that buying momentum is fading despite rising prices.

2. Failure Swings

Bullish Failure Swing: Occurs when the RSI drops below 30, bounces, forms a higher low, and subsequently breaks above its previous reaction high, offering a potent reversal signal.

Bearish Failure Swing: Unfolds when the RSI climbs above 70, pulls back, forms a lower high, and breaks its previous low, indicating an impending downward correction.

Strategic Refinements and Professional Pro-Tips

1. The 60/40 Bull-Bear Zones Pro-Tip

Regime Identification: In a strong bull market, the RSI often does not drop all the way to 30, but instead finds reliable support at the 40 level. Conversely, in a persistent bear market, the RSI rarely reaches 70, facing steady resistance at 60. If the indicator struggles to cross 60, you are operating within a bearish regime regardless of positive mainstream news.

2. Guarding Against the Overbought Trap

The Vertical Rally Context: During massive bull runs—such as the Nifty 50 surge in 2021—the RSI can remain above 70 for weeks at a time. Trying to short a vertical market simply because it is "overbought" is a classic retail error; in strong trends, overbought often just means "very strong." Always wait for a failure swing or divergence before betting against a powerful trend.

Key Takeaways

Measure Velocity: Use the RSI to quantify the speed and size of price changes on a 0-to-100 scale.

Respect Trend Regimes: Watch the 60 and 40 levels to identify whether the broader market is trapped in a bullish or bearish structural regime.

Master Failure Swings: Utilise RSI failure swings and divergences rather than relying solely on basic overbought/oversold line crossovers.

Demand Multi-Tool Confluence: Always cross-reference RSI readings with underlying price action, moving averages, and key support or resistance levels.

Next: Moving Averages