Classifying "Heartbeat" is maddeningly difficult. Gentle drums, plaintive progression and a cool atmosphere paint it as deep house, but the tempo is way too fast, bringing it closer to the sped-up excitations of Deep Teknologi. It's also a full vocal featuring the sultry gasps of London chanteuse Terri Walker, which might place it in the R&B column—if it weren't for the cavernous reverb. The vocals sound timeless, or maybe just stylishly-dated, that intoxicating cocktail of detached vulnerability and heated breath that screams glamourous '80s. The fantastically limber wordless ad-libs burn with a decidedly disco passion, and the mid-song shoutouts (altered from "T Williams" to "Mosca" for the remix) and elastic bass bring things full circle back to contemporary UK house and other pirate musics. A lot can happen in eight minutes, and "Heartbeat" is the epitome of a slow-burner, growing brighter and brighter until everyone on the floor is blind when its end finally comes.
Night Slugs associate Mosca takes the vocal and brings it much further in line to its spiritual home in the land of UK Funky, but still far from traditional. It starts off in lounge mode, all brushed drums and cool-blue piano chords: Terri Walker as jazz singer. Things quickly get strange, as violins seem to play themselves in circular paths and the wordless chorus drops off of the song completely, replaced by a gut-punching bassline. And when he cuts up Walker's vocal and juggles the nonsense syllables with his bare hands, captive audiences will turn to putty in his hands—acknowledged even by Walker herself as she endlessly moans, "Oh Mosca, you lift me higher."