It's a vibe.
The Massives have history with amazing guest vocalists (Elizabeth Fraser from Cocteau Twins, Sinead O'Connor, Hope Sandoval, Martina Topley-Bird, Tricky, Damon Albarn from Blur, reggae great Horace Andy, Tunde Adebimpe from TV on the Radio, etc.) after losing their original singer from the "Blue Lines" era, but IMO their greatest vocal contributions have come from Tracey Thorn of Everything But the Girl/Marine Girls fame. (Marine Girls are a total discovery, BTW).
Kind of a typically incredible Massive Attack puzzle assembly: the hi-hat/bass/"wah" guitar are all sampled from the intro of James Brown's "The Payback," with a completely new song/melody written by Tracey (in which gender roles are reversed, amazing) ladled over the top of that.
More medicine than music.