Hero Mode starts when the player completes all the goals of the level, and there are still moves left.
During Hero Mode, Cropsies do not reset their bonuses. There are even some additional bonuses thrown in. So you can get a lot of Cropsies compared to the normal playing.
In Rancid levels, there is no Hero Mode. Instead, each move is turned to 6,000 points(6% growth). The points are related to the number moves provided for the level.