How does i-Stop work?

The i-Stop function on your Mazda enhances fuel economy. It reduces emissions by automatically shutting off the engine when the vehicle is stopped, for example, at traffic lights or in stationary traffic, and then restarting automatically when the driver presses the clutch (manual) or brake/accelerator (automatic) again.
With a manual gearbox, the car will only go into i-Stop mode when it is in neutral gear and does not stop with the clutch pressed. With an automatic gearbox, the car will decide
When to stop the engine, which can even be from 25mph on the latest models.
This system contributes to reduced fuel consumption, fewer exhaust emissions, and eliminates the noise of idling when the engine is stopped.
This function will only operate under the right conditions. Due to the system being incredibly intelligent, it will not allow the vehicle to ‘switch off’ if it believes it will not be possible for it to restart. Please be aware that i-Stop may not always function at every idle. If the vehicle experiences high current draw from the 12v battery then the i-Stop may not activate. 
In the owner’s manual, it lists the conditions in which i-Stop will operate, which are:

• The engine is warm, and the car has been driven for a certain period
• The engine must be first driven; it will not activate on idle after startup
• The battery is in good condition
• The driver’s seat belt is fastened
• Windscreen defrost is off
• The temperature control for the air-con is set to a position other than max hot or max cold
• The vehicle's interior temperature and the set temperature are nearly the same
• The brakes are applied sufficiently
• The steering wheel is not being operated