After it took more than $150million at the US box office, but not much critical acclaim, the question on the movie industry's lips isn't so much "What Is The Matrix?" but "Why are so many people going to see it?" Could it have been the heady cocktail of Keanu Reeves' hoarse-voiced emoting and a hotly awaited follow-up to the Wachowski brothers' Bound? Was it the metaphysical themes appealing to cinemagoers' long-dormant spiritual side? Or - if you want to be pragmatic about it - was it just hordes of crazed Star Wars fans, driven to a state of near-unbearable anticipation by endless hyping of The Phantom Menace, ready to gorge themselves on pretty much any computer graphics feast that came along?

Or, then again, it could have all been due to The Matrix's unusually tempting trailer. As well as establishing the film's moody 7-Up ad lighting scheme, and that the action will, at some point, require "guns - lots of guns", the promotional clip for The Matrix offered a tantalising glimpse of what the film-makers have dubbed "bullet time", the distinctive special effect where, moving in a weird, almost dream-like slow-motion, Keanu limbo-dances beneath the line of fire of someone shooting at him.

This fancy camera trick is in fact a variation on the fairly well-known special effect known as time-slicing - a technique popularised in adverts for Capital Radio and The Gap, and employed to great effect in Vincent Gallo's otherwise ultra-low-tech Buffalo 66. In each case, an object appears frozen in time, and the camera circles around to show it in three dimensions - whether it's a head in mid-explosion or a casually-trousered swing dancer in mid-air.

Basically, time-slice works like this. All moving-image photography relies on the phenomenon known as "persistence of vision": a film camera pointed at its subject takes a rapid succession of still images, 24 of them per second, and on the cinema screen they blur into the impression of constant motion. More sophisticated cinematography is conjured up with tracking shots - the camera can jolt around like the Shaky-Cam employed in The Evil Dead films, or just lazily drift past, like every single shot in Armageddon. Again, the movie camera is actually taking a long strip of still images in rapid sequence, all from slightly different vantage points. Then, when they're played back, your brain naturally interprets them as the camera moving past the scene.

The essence of time-slice photography is that, instead of using a single moving motion-picture camera, it uses several still cameras, all of which take pictures simultaneously. If you take those pictures, and show them in sequence, from left to right, as if they were the individual frames of a movie, it creates the illusion of "virtual camera movement" - as if your viewpoint was physically roving around the object.

Obviously you're accustomed to seeing single, captured moments in normal still photographs, as well as the simple optical cinematic effect of a freeze-frame; but here the camera appears to be moving around, so clearly time must be passing. Yet the subject remains hanging there in space - oddly, inexplicably frozen in time.

The effect used in The Matrix is similar, but a bit more complicated. To keep the action going, the "bullet time" team didn't actually fire their 120 cameras simultaneously, but fractions of a second after each other, creating super slow-motion instead. A 3D simulation was used to position the cameras and trigger their exposures - and because, in most of the sequences, the cameras circle the subject almost completely, computer technology was used to "paint out" the cameras that appeared in shot on the other side. The cameras at each end of the row were standard movie cameras, to pick up the normal speed action before and after.

Similar effects were employed in fight scenes - in conjunction with more traditional wires and pulleys - which have Keanu Reeves tussling in mid-air with lead bad guy Hugo Weaving. But, like Terminator 2: Judgment Day, which astutely sprinkled its expensive morphing effects throughout the action to give the impression that more were used than actually were, the same old-fashioned methods underpin Reeves' fabulous kung-fu scenes with Laurence Fishburne.

The Matrix crew admit they were aware of the time-slice effect's earlier incarnations - for instance, Michel Gondry's Like A Rolling Stone video for The Rolling Stones. But many of its roots can be traced right back to a British photographer, Tim MacMillan, who says he invented the whole thing way back in the early 80s when he was a student at London's Slade School of Art, trying to create a photographic version of Picasso-style cubism.

"In a cubist painting, you get different angles of the same thing simultaneously," MacMillan explains. He built arrays of box cameras to try and capture this on film before realising he could also apply it to motion pictures. Over the past 15 years, he refined his discovery and shopped it around to various natural history camera units and advertising agencies, coming up with the term "time-slice" so they'd have a snazzy name for it when it was on Tomorrow's World in 1993.

MacMillan isn't too impressed with The Matrix ("It's one of those typical things - you work hard to get it accepted, and then loads of people do it") - even though, for once, it's an effect that neatly fits the film's mind-over-matter plot. But, since motion pictures began in the Victorian era, with Edward Muybridge's pictures from sequential still cameras of running men and galloping horses, Tim concedes even he may not have been the first. "You think, maybe there was a mistake one day, and all Muybridge's cameras went off together, and maybe on some shelf there are these glass plates of this horse, time-sliced, by Muybridge".

The Matrix is released on Friday 11