It uses the of lines of the closing price to determine band width. This has the effect of measuring around the current trend, rather than measuring for changes in trend.
In true TradingView spirit, the author of this script has published it open-source, so traders can understand and verify it. Cheers to the author! You may use it for free, but reuse of this code in a publication is governed by House Rules. You can favorite it to use it on a chart.
A list of Paid indicators: