In the event of a 51% attack, the attacker can reverse all transactions from the block they choose to start their attack from. If they started mining their own blocks from say a 5 block deficit, when they finally overtook the main chain and became the new longest chain, all transactions in those 5 blocks plus in any subsequent blocks mined since the attack started, would no longer be part of this new chain (unless the attacker had also mined them). Since they are no longer part of the new main chain, this means they never happened and so effectively would be reversed.
Your words seem logical but I took the quote from this text:
Even in the event that an attacker gains more than 50% of the network's computational power, only transactions sent by the attacker could be reversed or double-spent. The network would not be destroyed.
You can find the text on this page ---->
https://bitcointalk.org/adrotate.php?adinfoBut what happens after the attack ends and returns to the longer "real" chain? Only attacker transactions are canceled. Which means it's the only one that can be reversed.
if there are any mistakes in the text above I wish theymos modified it because it is considered a source of information.
