Here is the FAQs by Zynga about fighting system in Mafia Wars:
1. What are the changes that have been made to fighting in the game?
We have upgraded the technology used for fighting, to make it faster and more responsive. We have modified the code that controlled and enabled fighting. The rules of fighting have not changed. Fight logic, damage calculations, one-on-one fights, etc. will continue to work exactly as they did earlier.
2. What are the benefits of the changes in fighting and why are they required?
The new fighting technology allows you to fight faster, hit harder and tirelessly. The biggest benefit of the new technology is that the responsiveness of the game doesn't suffer when players are under heavy attack from others. The changes allow you to play the game irrespective of other players attacking you.
This technology improves the game responsiveness across the board when there is a lot of fighting going on.