The indicator uses the VIX as it's input.
The indicator uses the spread between the Call on SPX / SPY and the Put .
This is pulled from CVSPX and PVSPX.
When and put/call reaches a critical level, such as the levels present in a crisis or a sell off, the line will be green. See Sept 2015, 2008, and Feb 2018.
This level can be edited in the source code.
As the indicator is based on Put/Call, the indicator works best on larger time frames as the becomes a more discernible measure of sentiment over time.