When combined with SRAM’s compact (50/34-tooth) cranksets, the WiFLi derailleurs – which can accommodate cassettes from 11-26t all the way up to 11-32t – can offer the low-gearing options of a triple crankset without the weight penalty.
Since SRAM don't yet have Red- or Force-level cassettes to correspond with the WiFLi derailleurs (the lowest gear option on the new Red group's XG 1090 cassette is 11-28t), they're subbing in XX mountain bike cassettes until the road equivalents can be brought to market.
The SRAM Red WiFLi derailleur uses carbon inner and outer cages, titanium fixing bolt and spring, and ceramic bearings. It weighs 167g, compared to 151g for the XG 1090. The Force model uses a steel bolt and spring, and weighs 190g. Both feature SRAM’s new pulley design, which they claim quiets the system compared to older models.