You're thundering down a muddy descent, weaving around trees and dodging rocks, when all of a sudden The Log From Hell looms in your path. You have a fraction of a second to:

  — Lock up the binders, slalom to a halt and bellow profanities while you slowly dismount, pick up your bike and clamber over.

  — Try a bunny-hop that may send you sailing over the log — but could land you in the ER.

  — Smoothly dismount at speed and hurdle the log in a single bound, leaping back onto the saddle and speeding off like Zorro in pursuit of a bandito — without shifting out of the big ring.

  If this doesn't sound like mountain biking, well, no wonder. This is cyclo-cross. And while not every cyclo-crosser is a top mountain biker, most top mountain bikers are cyclo-crossers. 'Cross has served as off-season preparation for a galaxy of road stars, too.

  Why does 'cross appeal to fatheads and roadies alike? Because it makes both stronger and more skillful — and it's more fun than doing the Tour de Wind Trainer while watching "Days of Our Lives."

  So get off the couch, cobble together a bike and practice the three basic skills you'll need to pass Cyclo-cross 101 — getting off the bike, carrying it, and getting back on. While you're at it, grab a copy of Simon Burney's Cyclo-cross, available from Velo Press. This article is only a catechism; Burney's book is the bible. My copy's been passed around more than Paris Hilton.

  Dismounts come in three flavors — run-ups, roll-ups and full-tilt gut checks:

  — Run-ups. Shift into the gear you'll need at the top as you approach the hill, hands on brake hoods or bar tops. Swing your right leg over the saddle; move your right hand to the top tube, just in front of the seat post; then unclip your left foot — putting your weight through your right arm onto the top tube — and hit the ground on your right foot.

  Once you're afoot and running, flip the bike onto your shoulder by the top tube or the down tube, depending upon your size and style. More on this later.

  — Roll-ups. On an almost-rideable hill, stay on the bike until you start to lose the momentum you'll need to hoist it on the fly. Swing your right leg over the saddle, pushing down on the left pedal, then unclip at the bottom of the stroke and jump off. Run a few steps with your hands on the brake levers or bar tops, then shoulder the bike and beat feet.

  — Full-tilt gut checks. Many promoters use barricades to add difficulty to flat courses. Technique is at a premium during these high-speed dismounts; you can either gain ground, or lose skin.

  Start off slowly, with your hands on the brake levers, then add speed. This is why 'cross racers often reverse their brake levers — rear on the left, front on the right, and add top-mounted brake levers for security's sake. It's soothing to be able to modulate your speed as you charge toward a 16-inch-high wooden barricade in a half-dismount, with your left hand on a brake hood and your right on the top tube.

  As you approach, swing your right leg over the saddle, between the bike and your left leg, and slightly forward. Place your right hand on the top tube and lean back, with your weight through your right arm. Wait until the last possible second, then unclip your left foot and land on your right.

  Lift the bike with your right hand; your left remains on the bars or brake hood to keep your wheels straight. Hurdle the barricade, set the bike gently on the ground, return your right hand to the bars and leap into the saddle.

  If conditions are particularly heinous, or if you're nervous, you can unclip your left foot and approach a dismount with it simply resting on top of the pedal. But premature unclipulation risks sliding off the pedal on bumpy ground, or worse, clipping back in by accident — leaving you locked into one pedal as you barrel into a barrier at 20 mph. A well-drilled, last-minute release is far preferable.

  Keep your cleats and pedals well maintained, spraying them with cooking spray or aerosol lubricant on filthy days, and practice, practice, practice.

  Shoulder the bike for extended runs, particularly when the going gets muddy. Pushing it through the goo will gum up the works faster than a peace creep in the Pentagon. Some 'crossers like to set the bike down between hurdles erected in series, running with hands on bars and top tube. But the repeated lifting can wear you out.

  There are two ways to pick up the bike, and two ways to carry it (mountain bikers whose bikes' main triangles are cramped by sloping top tubes may have to try a third).

  — Picking it up, boss. If you're tall, try grabbing the top tube and palming the bike, shot-put style, onto your shoulder. Snake your right arm around the head tube to grab the left brake hood. Vertically challenged? Try using the down tube to hoist the bike, then reach under it to the left drop. Mountain bikers may have to reach over the top tube to grasp the down tube, then carry the bike like a portfolio (try resting the saddle tip on your shoulder).

  Keep your shoulder closer to the head tube than the seat tube for a comfortable, upright carrying position (a shoulder pad liberated from a woman's sweater or blazer helps, too). Lean forward into run-ups, taking short, quick steps; if you must run down a tricky slope, lean back a bit. You can open your stride somewhat for a flat run.

  — Putting it down, boss. Hold the bars with your left hand, then take the bike off your shoulder with your right hand on either the top tube or the down tube. The latter lets you sort of shrug the bike off and into your hand, and may feel more natural. But set it down gently; a bouncing bike is nothing you want to leap onto if you hope to reproduce.

  Put both hands on the bars (tops for the flats, drops for descents, levers for everything else). Leap off your left foot, throwing your right leg over the saddle; land on your right thigh, then slide atop the saddle. You'll probably stutter-step for a while, but strive for the clean, one-step leap from left foot onto thigh — it's faster and safer.

  Stab your right foot down for its pedal (a proper dismount should leave it near 12 o'clock). Clip in and stomp on it, then clip your right foot in as its pedal comes around. Haul ass. Repeat as necessary.

  So much for Cyclo-cross 101. Check out Burney's book for advanced studies of everything from technique to technology — but don't forget that every education needs real-world application. Go out and get muddy.