William Blau's construction smooths the bar-to-bar price change twice, does the same to the absolute value of that change, and takes the ratio. The result oscillates around zero and is far less jagged than raw rate-of-change.
Because it is centred on zero it reads naturally as a directional momentum measure: above zero means momentum is positive. It is used with a signal-line for crossovers and for zero-line crosses.
Double smoothing buys that cleanliness with substantial indicator-lag. TSI turns noticeably later than price and later than single-smoothed measures, which makes it more suitable for position-level context than for entry timing.
Related: momentum-indicator, rate-of-change, indicator-lag, signal-line, macd