The bypass starter valve is what people ignorantly call a choke. It serves the same purpose as a choke, but it's more efficient and operates opposite of a choke.
A choke is a butterfly valve in the mouth of a carb and when closed it chokes off air to create a fuels rich mixture for starting a cold engine. Such as is it's operation, the throttle has to be opened to speed up the engine to make it run with the rich mixture entering the cylinder. In essence the throttle is being opened to lean out the rich mixture entering the cylinder. It's difficult to set a choke to achieve the correct fuel rich mixture for starting a cold engine.
The bypass starter valve opens a passage within the carburetor that lets a metered amount of fuel and air to by pass the throttle butterfly. This lets a fuel rich mixture into the combustion chamber along with a metered amount of air to provide easier starting and a fast idle without need to open or feather the throttle to keep it running as it warms up.
The Honda TRX400EX uses a carburetor with a butterfly choke, but the choke plate is spring loaded so as a gulp or air is entering the cylinder, the plate springs open to help prevent over choking. In cold weather you choke it to get it started and shortly after starting you can start opening the choke to prevent it from choking to death and fouling the spark plug. If you do not wait fro full warm up and start riding with the choke partially on, youi run the risk of fouling the plug, wasting fuel and in certain cases, carburetor damage.
With the bypass starter, you don't want to open the throttle when trying to start cold - opening the throttle reduces intake vacuum and renders the bypass starter ineffective or useless. Best way to start an engine with a bypass starter is to not touch the throttle and engage the starter, it should start and fast idle at a preset fast idle speed about 2000 to 2500 RPM. Just let it fast idle while it is warming up and you are preparing to launch. When ready to take off, close the starter valve and it it is not warm enough to idle, when the engine slows enough to shift into gear, shift, open the starter valve and start riding - as long as the throttle is over 1/4 open, the bypass starter is not doing anything. When the engine is fully warm and it is convenient, close the valve and continue to ride.
Now every engine has a slightly different cold start routine. On my Honda CRF with a Keihin FCR carb with an accelerator pump the starting procedure is to open the starter valve and if the air temperature is around freezing, pump the throttle 3 times slowly, press the starter button and it will start runs a few seconds and die. You then pump the throttle twice and restart, it will usually idle about 5 seconds and die. You then pump the throttle once and restart, at this point it may idle 10 seconds and die in which case you pump the throttle once and restart or it may idle down almost to the point of dying and then accelerate to the fast idle speed. At his point I let it idle while I put on my riding gear (compression shorts, thigh high socks, kneeguards/braces, pants, MX boots, jersey, chest protector, drink system (if needed) helmet, goggles and gloves) I then see if the bike will run with the starter valve closed. If it dies, I open the valve, get on, start the engine and start riding. About 1/4 to 1/2 mile into the ride, I find a spot to reach down and push the starter valve closed. From this point on the valve is not needed for a restart unless the engine is allowed to cool for about an hour, then it might be needed if a couple pumps on the throttle is not sufficient to get it restarted.
So now you know what a bypass starter is and how it works. It's still affectionately called a choke by most who do not know how it works or what it is. They just know using it aids starting a cold engine.
