What is i3s?
i3s is known as Idle Stop Start System.
How i3s Works:
This technology is developed by Hero MotoCorp and implemented in Hero Splendor iSmart bike. This system automatically switches off the engine and switches back on when you press the clutch. When you stop the bike the i 3S system puts the engine into hibernate or sleep mode.
The Idle Stop Start System turns off the engine after waiting for about 5 seconds for e.g. at a traffic signal. The system then restarts the engine automatically as soon as you press the clutch without having the need to follow the routine procedure to start. This prevents running of the engine in idling when not needed and thus improves mileage. some more information on company website.
See How i3-S Works here: