What does it make when the Choke butterfly (valve) is closed
The air which penetrates into the carburetor is regulated first by the choke butterfly then by the throttle. Activating the choke means closing the choke butterfly. So when the cylinder goes down, it creates a vacuum and aspires a lot of fuel with few air. This is called a rich fuel / air mixture. The reason for this rich mixture on cold temperature is that the fuel does not vaporize easily so you need an overall fuel ratio.
Source: Youtube Channel : engineering explained

What is its function ?

The Scintilla, also known as thermal delay switch, is related to the choke system
. Due to the compression of a cranking engine, the choke valve (or butterfly) needs to be kept close by some means. This is why a choke solenoid (also referred at as electro magnet or fuel weakener solenoid) is used in order to keep the choke valve closed under certain conditions. But as this "stucked close" function is useless by normal temperature, the choke solenoid circuit is activated through a bi-metal switch (also called Otter Switch) which closes the circuit below 4° Centigrade.
Later, when the engine has cranked and reaches a certain temperature, the choke valve has to be released and this is what the Scintilla is used for. On older cars this was performed by an oil pressure sender put in cirsuit with the Otter switch. When the engine oil pressure started to rise the oil pressure sender would make the Otter switch brake contact with the choke solenoid and the choke butterfly would then be released.
But in certain cases this technique proved too quick and this is why the Scintilla was introduced. This has a heating coil which takes a while longer to warm up before breaking the circuit thereby giving the engine time to sort itself out.
Source: Tee One Topics issue # 5

Scintilla and Otter switches
The Scintilla contains a bi-metal strip with an enclosed heating coil. The coil heats up and at a pre-determined time causes the bi-metal strip to switch, which deactivates the choke solenoid.
The otter switch - wich sits at the top of the butterfly housing - does not contain it's own heating coil and relies on the external temperature (ambient temperature) to heat the bi-metal strip and activate the switch.
Source: The Rolls Royce Forum - England

A bi-metal switch is
A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases steel and brass.
Source: Wikipedia

Testing the choke solenoid condition
The simple test to see if the system is working obviously done in very cold weather is to set the choke with the accelerator in the normal manner with the ignition on and try and open it with your finger using the small lever at the side of the air intake. If all is well there will be considerable resistance to your finger’s efforts. The whole setup incidentally can be over-ridden by flooring the accelerator which simply forces the choke open. Such a facility you would use in the event of flooding!
Source: Tee One Topics issue # 5