Alright, I think Ive understood it.
Now the question:
What is the advantage in re-writing the bitcoin blockchain? Why should someone do this? Its just theoretically...
The blockchain is the list of transactions that have occurred.
If you re-write the entire blockchain, then you get to eliminate ALL transactions that have ever occurred AND you get to keep ALL the bitcoins for yourself.
If you only re-write a recent portion of the blockchain (such as the most recent 30 blocks or so), then you can eliminate a transaction were you paid someone. In that way, you could pay for a product or service, and then after the product or service was provided, you could remove the payment so that it never existed and you have your bitcoins back.