In the summer of 1966, the Beatles had just recorded Revolver, rock's first full-on foray into what a band could pull off in the studio. But they were still every bit a live, coming-to-your-town touring band when they trekked off for world tour number three. It was a tour that, in the wake of John Lennon's claim that "we're more popular than Jesus", would lead to record burnings and death threats in America's Bible belt.

Beatles obsessives have long talked about what happened on that tour, and in particular what happened at a gig in Memphis. Someone shot at the band, goes one theory. A car backfired, runs another. The general consensus, though, is that someone lobbed a cherry bomb, a powerful type of firework, at the stage, while the Beatles performed their second set. Depending on who you listen to, or which web chatroom you log on to, the Beatles stopped short - or carried on as though nothing had happened. Some people say the band were frightened by the explosion - they had mistaken it for a gunshot, each looking around to see if one of them had been shot down. Whatever the truth, collective decisions rarely come faster. As Lennon said, that was it. Last tour. We're done here.

And then, late last year, word started going around: a tape that had long been hoped for, but no one really thought would ever turn up, would soon be up on the web. It turned out that two teenage girls had lugged a portable tape recorder to the Memphis show. There were already plenty of 1966 shows available as bootleg recordings, including a number from Tokyo in near-perfect fidelity for the era. But the Memphis gig was the stuff of fantasy.

If you collect bootlegs, as I do, you live for that moment when incredulity gives way to wonder. The tapes of the 1966 Tokyo gigs don't inspire any wonder: they're a good indication of how poor the band was throughout much of their final world tour. But when I first heard what has been dubbed the "Cherry Bomb Tapes", after tracking it down online, I heard a group raring to go. These guys were up for it. However, once we get to If I Needed Someone, swagger turns to humility mighty fast. Someone does indeed set off a cherry bomb, or some kind of backyard explosive, and the men of the moment blast off into double-time, Lennon positively flogging his rhythm guitar.

With audible proof of the explosion comes debate. What, for instance, would have happened if that cherry bomb had never gone off? Touring was still a possibility for the Beatles, pre-cherry bomb. That firecracker is the sound of a decision, a ne plus ultra moment for a band that was already contemplating a seismic shift in how they were going to do business: in the studio, with rock'n'roll taking life as collage art, rather than the stuff of teenybopper caprices and the night out at the baseball stadium.

The Beatles' alleged telepathy gets a lot of play in the assorted versions of their story - like George Harrison's oft-cited remark that he wasn't sure what "we" thought about God yet, as though thoughts passed from Beatle to Beatle without need for articulation. But to have the life all but scared out of you - to share that experience with three other people, and arrive at the same conclusion - isn't so much telepathy as basic humanity coming to the fore. The band who get terrified together tend to retreat together, especially when their art is better served in doing so.

Let's consider Pepperland, and how the distance from the band's final tour to the experimentation of Strawberry Fields Forever and Penny Lane was bridged by what went down in Memphis. Once that cherry bomb went off, with no need to worry about recreating their studio work on stage, the Beatles were free to tear loose. Before long, the rest of the heavy hitters would follow suit. The Stones ended up with the jumbled Their Satanic Majesties Request, while Brian Wilson, forever in heated competition with Lennon and McCartney, soon found himself going mad. Studio fever, almost. But the demarcation had been established. Touring was no longer at the heart of what it meant to be a rock'n'roll band. If what you did in the studio was big enough, you didn't have to worry about whose town you weren't going to next. It was all about the vinyl now.

The best bootlegs function as great lost masterpieces: the 1966 Bob Dylan show in Manchester was the classic live recording of his career, but languished in the vaults for 40 years before getting an official release; check out the Stone Roses live at Glasgow Green in the summer of 1990, when even their staunchest followers were wondering if they could still cut it. Most bands, if they're lucky, have one great live album. If you want to know what those bands were really like at their best, you have to go underground.

The Beatles didn't even really get their one classic live album, unless you count the patched-together Live at the Hollywood Bowl, from 1977. But the Cherry Bomb Tapes are much more than just a live recording. It's not especially uplifting to hear a band playing as though they have just been shot at, and a lot of listeners will find their pleasure quotient ratcheted down by the sound quality. But what you're really listening for is history, the sound of a collective, immediate decision. And while the historical cachet of bootlegs typically centres on their artistry, the Cherry Bomb Tapes are different. They're about history itself, a distillation of a tall tale into the here and now, folklore becoming tangible.