Apple File Vault & HD erasing I am at the point where I am about to erase the HD on my old iMac 2009 running El Cap before selling it.

I recently turned on FileVault to encrypt the volume mainly to see how it worked and also as an aide to what I thought was going to be a quick and secure erasure of that HD.

I have reached this point following these instructions on MacWorld:

With a FileVault-encrypted startup disk, you can restart into OS X Recovery and launch Disk Utility to erase the volume. However, before erasing, you need to select the disk and then choose File > Unlock “volume name”. Enter the password for any FileVault-enabled user account, and the disk is unlocked and can be erased.

When I choose Erase, the next window gives me the warning:  Are you sure you want to erase the partition “Macintosh HD” and create an encrypted partition?  Warning:  Files on this partition will be encrypted using this password.  If you forget the password, your data will be lost.

It's either cancel, and then I can't erase my HD or continue.

Not sure what I am doing at this point and could use some help.  

Am I setting up FileVault for the next owner of this iMac?

What does the message in this window really mean?

Many thanks.
Once FileVault is done with the encryption, you can "cryptographically" and fairly instantaneously erase the drive by destroying the keys to decrypt.

I wouldn't bother setting up the next encryption. To do that, boot to recovery and do not unlock the disk.

Just repartition the drive.

That wipes the keys that the passphrase would have used to decrypt the drive. It's now cryptographically erased.

At that point, you might install a clean OS and shut down the Mac when the "welcome to Mac" screen prompts you to create an account and set up the Mac.

Shutting down then lets the next person know the Mac works, and you know it's securely erased.