A True Strength Index (TSI) value below zero signifies a bearish market condition, indicating that the momentum of the asset is in a downward trend. The TSI is a momentum-based indicator developed by William Blau to capture both the speed and the direction of price changes, smoothing out the noise often associated with other oscillators.

When the TSI value drops below zero, it suggests that the average of the recent gains is less than the average of the recent losses. This negative value reflects increasing selling pressure and can be interpreted as a signal to sell or hold off on buying, as the asset may continue to decline in value. Traders often view a TSI below zero as confirmation of a downtrend.

Moreover, the strength and duration of the negative TSI value can provide insights into the momentum of the bearish trend. A steep decline or prolonged period below zero typically indicates strong and sustained downward momentum, while a shallow or short-lived dip below zero might suggest weaker or temporary bearish conditions.

In practice, traders may combine the TSI with other indicators or analysis techniques to improve the reliability of their trading decisions. For instance, confirming a bearish TSI signal with volume analysis or other trend indicators can help traders avoid false signals and better understand the market dynamics.