It is based on the fact that the upper range limit of the private key used in the puzzle is known. A securely generated private key would not be vulnerable even

It is based on the fact that the upper range limit of the private key used in the puzzle is known. A securely generated private key would not be vulnerable even if its public key is known. The second post on this thread[0] has a helpful chart that makes it easier to understand. 0: https://bitcointalk.org/index.php?topic=5218972.0 https://bitcointalk.org/index.php?topic=5218972.0