image sources: instaforex.
It is one of the best and very informative Oscillator Indicator, invented by William Blau. The main future of this indicator is its calculation is not based on Price. But tick density.
If the market moves down and the TVI bar has changed to Red, it denotes Sell. In reverse it denotes Buy.
The color change alerts time to enter the market.
Ref: instaforex