Whenever the state machine moves to state `S2` (super-state), it will automatically transition to its first sub-state, that is `S3` (`S2` is a state machine itself and `S3` is its initial state).
We can notice that state `S3` only has a self-loop trasition that is triggered by event `EV_2`. However, if the HSM receives an event `EV_1` while being in state `S3`, the event is handled by state `S2`, which is a super-state of `S3`.
