in Passo having push start function,
there is actually no push button, but a sort of plastic handle, which rotates like a key

and there is a place in it to insert key as well 
from factory, passo comes with
- a key (with immobilizer chip in it)
- a remote(immobilizer) and in it a small key with no immob chip (used to unlock door only)
if remote battery is weak, then you can open doors by the small key, take the remote closer to ignition push switch, and it unlocks the steering to start the car.
if you dont have remote with you, then you can also start the car wit the key(with immob chip buit-in) by inserting it into the push ignition switvh.