Chakra stolen is Chakra they have not yet used or leftover Chakra after the enemy has cast a mystery.

It is not an interrupt of any kind so if a ninja starts their Jutsu they have already drained this Chakra from their Chakra Bar. It means you can't steal that lol

2nd turn = 40chakra.
If the enemy uses all 40chakra or starts their 40chakra jutsu before you hit them...
you will not be able to steal their chakra even if your mystery attacks before theirs.

Turn 2 =
Torune casts mystery
and enemy casts rasengan. (enemy now has 0chakra even though the rasengan has not hit)
standard attack to steal chakra = 0chakra (enemy had 0chakra at this point ^)
End Result...you stole no chakra