A number of conditions can prevent your dirt bike from revving to life either through the electronic start or traditional kick start. In this case, the only alternative other than walking it home is the bump start.
Bump starting a dirt bike or just about any motorcycle for that matter is similar to bump starting a car with a manual transmission. You need speed. A nice long hill is your best bet but with any luck a helpful friend or two can probably push you fast enough to make the magic happen.
Most reasons for a bump start fall on the rider. Just like mishandling the clutch stalls a car, mishandling the clutch on a dirt bike shuts the engine off. But if you're already ripping around the track or cruising through the woods usually the bump start happens by chance as you work the clutch and the engine kicks into gear.
On the Motocross track, stalling the engine often happens after a jump when the rider doesn't coordinate hitting the break to prevent an air wheelie and the clutch at the right time. There's nothing worse than a silent engine as you fly through the air.
But once on the ground, you've already got the necessary speed and it is pretty easy to put the bike in the appropriate gear and let the clutch out once the tires get rolling. If you're in a race you'll lose some ground but it's better than rolling limp to the side and certainly better than crashing.
Some dirt bikes have batteries, though most do not, so a dead battery (the primary reason you'd bump start a car) is another possibility. If you've got issues with the spark plug or engine compression issues you can kick all you want but the only way you'll get to the gates is a bump start until you fix the problem.
Lastly, a flooded engine also prevents a quick restart after a stall. You can either wait it out or bump start your bike back to life, however you'll need a runway's length to pop a flooded engine.
How to Bump Start a Motorcycle or Dirt Bike
In case you hadn't noticed, a dirt bike is a motorcycle so if you can bump start one you can bump start the other whether it's a touring machine or street bike. A regular motorcycle typically requires a bump start thanks to a dead battery or some other starting malfunction. Most street riders would opt for the usual jump start from someone's good battery, though.
If you're alone you'll need a hill since running alongside the bike, hopping on and popping the clutch with enough speed to roll the engine over is an unlikely scenario. So find the jump with the longest lead up on the track or the longest hill on the trails. Whether you're using the pull of gravity or getting a push start from others you'll need the speed appropriate to riding in second or third gear.
Hold the clutch in as you coast along, click up to second or third gear and once you're up to speed, let go of the clutch. If you found the ideal hill for maximum speed you can probably just let go of the clutch handle otherwise release it slowly because you might stall again or lock up the rear tire. It may take a few tries before you get the hang of it so if you stall out, try it again.
It's that easy and now you're back to riding.
For an interesting alternative check out this fun video we found but based on the comments it's apparently not as easy as these guys make it look.