The + sign in the keyword suggests a hybrid approach. You use AV4 logic (intelligent frame analysis) to decide which parts of the US stream (raw data) to keep. This results in a "visually lossless" master—identical to the source to the human eye, but 70% smaller than uncompressed.
Because this term is often linked to unofficial or adult-oriented video hosting, it could mean one of the following: av4+us+video+extra+quality