Basic question answer - the flywheel is the round thing between the engine case and the clutch bell.

As far as stopping the engine, all of the above work to one extent or another. Of those mentioned my least favorite would be having to remove the air filter on some engine to plug off the carb, major chance of dirt entry. On some filters that have a 45 or 90 degree extension it is possible to either pinch it closed or push it over the carb venturi shutting off the air, however, at some point you with either knock it off or shove it into some spinning object causing wear allowing dirt to enter.

Next on my not favored list would be pinching off the fuel line, only because on some models the body must be removed first. If the body is already off it is my favorite way to shut the engine down, especially if its the last run of the day.

Foot, finger, screw driver, etc. to the flywheel. Fine for 12 and 15 sized engines - IF THEY ARE AT IDLE. I have seen some 21's and up leave some cuts on folks thumbs when they stopped them and didn't get the pressure on quick enuff, especially with thin flywheels. Never try to stop a runaway by stopping the flywheel.

Most of the time my favorite way to stop an engine is to just block off the exhaust stinger with my thumb or finger. If the exhaust has no where to go the engine stops rather quickly. Keep a rag in your pocket to use to shut it down. I forget who makes it but at least one company makes a pointed plug with shirt pocket clip that plugs the exhaust stinger rather nicely. If its a runaway this also seems to be the best way to stop it, but don't try to use a body part, use a plug, a rag, stick the stinger in the dirt ( least favorite option - but remember it s a runaway).

But the best way is to enjoy the run till all the fuel is gone.

Ed M.