There are two main ways to 51% attack a blockchain and try to reap profits from it:

* Send two conflicting transactions in rapid succession into the network. This is called a race attack.

* Pre-mine one transaction into a block and spend the same coins before releasing the block to invalidate that transaction. This is called a Finney attack.

VeriBlock exists to not only protect itself from these attacks, but also protect other altcoins and exchanges.

Race attacks and Finney attacks are not 51% attacks. VeriBlock only intends to protect from 51% attacks, not from race or Finney attacks.