Within 20 minutes of arriving at the first day of Helsinki’s Flow festival, I’ve seen some rapturously received Finnish afrobeats, an experimental violist playing gorgeous loops and drones in a dimly lit hall, spine-jacking techno (a bit much, at 5pm on day one, for all but three devotees), and a doomy art installation in a grand ex-turbine hall telling us exactly when we’re all going to die. Oh, and Suede and Wizkid are on later, for those who want their festival drink a little more straight-up.

In short, Flow are nailing the something-for-everyone model, offering genuine variety for each attendee. The ace up their sleeve though – the thing that stamps Flow’s identity on an otherwise punter-led choose-your-own-festival adventure – is their Balloon 360 stage, a steel circular amphitheatre at the far end of the site, above which floats a perfectly lit 40-foot-wide balloon and a wraparound video screen. Set away from the rest of the festival’s hubbub, and largely programming jazz, ambient and dub over the weekend, the stage is a perfect oasis of calm, and one of the most dazzlingly unusual festival arenas you’ll ever see.

It is also a perfect set-up, as the sun sets, for Nala Sinephro and her band to present their willowy take on melancholy-ecstatic spiritual jazz, all Alice Coltrane/Pharoah Sanders ambient lushness. Starting with a harp voluntary, Sinephro then moves to synth burbles and gently adds each member of her four-piece band to the pot until the quintet are moving balletically as one 20-limbed organism, waves of sound crashing and retreating over an ever-growing first half hour in which every note feels treasured and luxuriated in, no matter how big the build or deep the fall.

Slinking film-noir bass then takes the set in a different direction, as a hint of a full-rave 140-bpm banger lurks just beneath the surface. Instead of a drop, though, they settle into a loping grove, the band’s poise and precision masking their sheer force, and after an hour the set vanishes into itself in much the same way it started: pin-drop quiet, dreamlike and still.

Perhaps what’s most impressive, though, is that unlike some contemporary jazz where the emphasis is on the extremes – a million notes a second, feats of staggering technical chops – in favour of a full spectrum, Sinephro and her band here investigate the gamut of sound possibilities with real expressivity: of course, they’re expert players, but never showy, and a wistfulness washes through the set, carrying both anguish and ecstasy at different points when the synth or trumpet lines bubble out of the brew.

That level of nuance and grace, in such a beautifully unique setting, offers the performance a genuine sense of event; that this isn’t just the next cab off the rank on the festival line-up, but something really rather magical.