Nothing raises the ire faster of a rider than a dirt bike that refuses to start after a mid-moto crash or stall.
That's actually the genesis of a number of new swear words you hear on the track today.
Thanks to the modern adaptation of fuel injection most newer bikes don't have this problem but those still riding models manufactured around 2010 and older (or any carbureted dirt bike) can encounter a flooded engine which results when a bike is hot and you stall it.
That's not to say every time a hot bike stalls it won't fire right up again. In fact, it's not uncommon to hear from someone who has ridden for years with a bike featuring a hot start lever and they've never used it, don't know what it is or even how to use it. The hot start function is similar to a choke for a cold start. Whereas the choke allows more fuel than air (oxygen), the hot start allows more air than fuel.
Why Do I Need a Hot Start?
When a bike without fuel injection is running and suddenly stalls it floods the carburetor with gas because the engine briefly receives fuel after it stops firing. This throws off the air/fuel mixture preventing the fuel from igniting thus preventing the bike from starting.
It's similar to older model cars that don't have fuel injection. If you're old enough and have fond memories of playing around with your parents' car by pretending to drive and stepping on the gas pedal while they ran into the grocery store real quick you found out when mom or dad returned that you flooded the engine.
By pressing on the gas with the car's engine off you flooded the carburetor with fuel so you had to wait five or 10 minutes when the fuel drained back into the gas tank and the fuel/air ratio returned to optimal starting ratio. Cars didn't have hot starts back then and the fuel injection eliminated that problem for a new generation of kids, but thankfully the dirt bike manufacturers figured out a way to get a rider back up and running otherwise you might as well walk her back to the pits.
One example of a hot start lever
How to Use a Hot Start on a Dirt Bike
The hot start lever is located on either the left or right side of the handlebars next to the clutch or brake levers. It's about a quarter the size of the clutch lever and is small enough and out of the way so it doesn't interfere with riding. Pull it in just like the clutch lever (it doesn't have the same snap as the clutch or brake levers) or in some cases press or push it, then use the kick start. Don't pump the gas (roll the throttle). All that does is spill more fuel in to the engine, much like you did on your parents' old car. Once started, let go of the hot start lever and get back in the race.
It's that simple.
Restarting might require two or three good kicks, but in the heat of the moment as your place in the race drops from the Top 5 to out of the Top 10 and counting don't be surprised if panic sets in. It's normal but you need to remain calm. Take a few extra seconds, which will certainly feel like a minute, to ensure a full, strong kick on a compression stroke.
If you've raced enough times you've probably seen other riders in similar situations frantically trying to kick start the bike to no avail. This drains their energy and until they stop and get a solid, full kick the bike won't start.