Twenty years on we can only guess at the original effect. Subversion of ballet's rule-book now being almost the rule, it's no shock to see classically trained bodies doing things they didn't oughta - syncopating rhythms, kicking their heels, wiggling their bums. The joy of pushing the powerful body of Baryshnikov into such new tricks was that it went so deeply against the grain. In the slighter, supremely pliable body of the Royal Ballet's Tetsuya Kumakawa, somehow you feel it all comes rather easily, as if loose-limbed jazziness were his regular mode of moving, and classical containment the thing he had to work at. That said, his performance is stunning. The elegance of his spinning leaps, his immaculate landings, the humming-top blur of his long, fast pirouettes, provoked audible gasps round the house.
The point of Push, however, is the clever way Tharp flirts with the music - a ragtime by Joseph Lamb, followed by a symphony by Haydn, which itself pulls all kinds of pranks on classical expectation. Here is a composer after Tharp's own heart: rigorously intellectual yet bursting with fun. As the strings romp towards one of those inevitable classical cadences, Tharp can't resist beating Haydn to the post, having Kumakawa cheekily stand and wait, arms folded. Next time round, he spins on after the final chord. Hey, Mr Haydn, you wait for me.
The chiffony girls - Sarah Wildor and Darcey Bussell, and later Deborah Bull - come on suave and cool, but little madcap gestures keep breaking the surface like bubbles of laughing gas. Classical epaulement turns into shimmy-shimmy shoulders; Darcey extends a luxurious long arm halfway to arabesque then appears to be checking her watch. Behind them the girls of the corps de ballet have a whale of a time interlacing fiendish classical demands with moves from barn-dance and Broadway. They clearly adore dancing Twyla Tharp. The audience adores Twyla Tharp. Be as nice as nice to her, Sir Anthony Dowell. We'd like more.
The evening began with David Bintley's Consort Lessons - possibly a mistake, as this very lovely pure-dance piece, harking back to that other classicism, ancient Greece, got rather shoved aside in the event. The Stravinsky concerto for piano and wind is relentlessly severe, yet Bintley finds in its complexities the sort of lucid spatial logic that would have set Pythagoras's senses tingling. Sarah Wildor, hoisted in flight like an Icarus above the heads of four strapping athletes, is one of many frozen images that endure.
And no one walked out of The Judas Tree. Kenneth MacMillan made many dark ballets, but none more bleak than his last, only months before he died in 1992. Are gang rape and murder suitable subjects for dance? On this evidence, yes. MacMillan expects of his audience the sort of mature and self-searching insights that a high-court judge would like to expect of a jury. And no one's saying it's easy.
Enter the all-male world of a building site under lights. Unreal yet super-real, tribal instincts rule, and the Foreman (a slicked-back, thuggishly sexual Irek Mukhamedov) is chief. When a scantily dressed girl mysteriously appears and flaunts her elastic limbs (Leanne Benjamin's do things our creator certainly never intended), the inevitable happens. So? She was asking for it. Yet MacMillan asks us to go beyond moral certitude. Extraordinary as it seems, he actually asks us to admit to our own excitement.
This grim scenario sprang, not fully formed from the choreographer's tawdry mind, but from Brian Elias's highly potent score. It suggested itself. And as the music unfolds, bit by bit listeners find themselves, along with the brutish protagonists, caught up in the mounting frenzy of a primal voodoo only blood sacrifice will assuage. Some of the most stunning machismo dancing ever devised for the stage spurs the ballet on to its horrible end. And as we are left morbidly aroused, yet morally sobered, the lights of the half-built Canary Wharf wink familiarly over the scene. It's all very close to home.
Royal Opera House , WC2 (0171 304 4000), Tues. 'Push Comes to Shove' returns in July.