“There are people that hear it once and think: never again!” says Evan Parker of free improvisation, a musical style that some might compare to a jazz band falling down the stairs – and others find transcendental. “Then there are people who hear it once and say, ‘My god, what was that?’ But they creep back, because there’s something that’s connected for them. There’s a worldview involved that touches people.” This short film, as part of the Guardian’s series on underground music, featuring Parker as well as other free improv luminaries such as John Edwards and Eddie Prévost, gives you a six-minute taste of this worldview.

Play Video 6:17 Free improvisation: still the ultimate in underground music?

Parker has spent 50 years playing his saxophone without a song sheet, without notation and often without any idea of how a performance is going to evolve. I’ve seen the moment he describes unfold a number of times. At a Parker show in 2014, a clean-cut couple sat next to me watched the first 50-minute set of the evening in near silence, before turning to me to exclaim: “What the hell just happened?” An hour later they said: “Well, it’s something to tell them about at work tomorrow …”

None of this is to say that the music is elitist – in fact it represents the absolute rejection of the elite. The origins are disputed, but at some point in the late 1950s jazz players and modern composers who were repelled by the codification conservatism of their peers and broke free. There is no manifesto, no union or club you can join, just a shared worldview and an acceptance that no one is going to get rich from it. The claims of non-hierarchical band structures are not always borne out – you can’t deny the seniority of experience, or the person who organised the gig – but this is as close as you’re going to get to a music that reflects socialist values.

All of this makes it the ultimate in underground music. The music is simply too inaccessible for the mainstream, and no one involved is particularly interested in it anyway.

There are key recordings – Parker’s Topography of the Lungs, AMM’s AMMMusic – but for many of the musicians, the process of making the music is as important as the results. This creates a suspicion that free improvisation is simply music for musicians, that an audience can never get close to it in the same way a performer can. There is some truth in this. Occasionally at live shows, you feel as if you’ve intruded on someone’s private space, or that you’re watching scientists at work in the lab. But if you put the effort in and offer yourself to it, the shock you might feel at first will recede, and, as Parker says, you’ll creep back.