''Grab, grab, step, slap, cross shoulders, slap, 5-6-7-8, grab...''
The five members of 'N Sync, each sporting a chevron of sweat on his chest, advance in a line toward an imaginary audience. This rehearsal of stage moves for the 52-date North American concert tour that kicked off May 9 in Biloxi, Miss., is running late and, in contrast to almost everything else in their now astonishingly successful career, not going especially well. As 19-year-old Justin Timberlake pirouettes toward stage rear in the see-ya strut central to boy-band choreography, the growled word ''sucks!'' bounces off the stage and onto the cement floors of this skating rink-cum-concert hall called Lakeland Center, 40 miles southwest of Orlando. ''You gotta look at the slap,'' says determined if diffident choreographer Wade Robson, a lanky, peroxided dance prodigy who's all of 17 years old and came off Britney Spears' stage to share terpsichorean chores on this tour. ''The straight line on the verse has got to be straighter.''
JC Chasez, 23, is the first to reset himself at center stage (as cowriter and producer of ''Space Cowboy,'' the song they're staging with cowpoke gesticulations, he has a vested interest). The other four lads in the legendarily hard-working group, who've been literally put through their paces for hours, line up for another run-through wearing carefully neutral expressions, as Robson adds, ''It's only three minutes.'' Chris Kirkpatrick, owner of the highest voice and, at 28, the oldest but without a doubt most irrepressible 'N Sync-er, corrects him with mock exasperation: ''Three and a half minutes.''
The minuscule bubble of tension that briefly floated now pricked, the band goes back to the business of being the biggest pop-music juggernaut of the last...few months. In fact, over the last seven weeks they've rewritten the world records for speedy album sales, and now they're plowing toward the record for numbers sold in a year. Yes, they arrived as merely part of a pop-demographic landslide that sprouted with New Kids on the Block, flourished with Hanson and the Spice Girls, and more recently has steamrollered rock-rap acts like Limp Bizkit to take over the chart-topping positions hip-hop owned in the mid-'90s. But while at it, 'N Sync have leapt onto an entirely different plateau than their big-selling Jive Records label mates Britney Spears and the Backstreet Boys. True, those acts share the RIAA's new 10-million-albums-shipped ''diamond'' status with them, but from the moment 'N Sync's No Strings Attached hit the shops March 21, it's ka-chinged at a rate that makes the competition's sales seem a tad lethargic. The album's 1.13 million copies sold in the first day obliterated any previous going-like-hotcakes marks, as did its 2.4-million-units-sold opening week. ''This thing sold about 5 million copies in its first five weeks,'' notes Billboard charts director Geoff Mayfield, abandoning standard arithmetical chatter to add, ''That's just nuts it really is kind of scary.'' Mayfield acknowledges that third-week numbers were falling off, ''but then you got to the fourth week and it actually had this handsome increase the biggest Easter week we've ever seen.''