OK, you got it then, I believe. P3 is specifically built so that P3(xn)=yn for n from 1 to 4. The proof lies in its construction. I guess the sentence can be understood as “we know it works because we built it like that, however you may verify it yourself”
Replace the words “Convince yourself” with “You can verify” and it might make more sense.
Short and on point :)
That’s not possible because of the way disk encryption works. When you unlock an encrypted drive, it does not actually decrypt it - that would take way too long and leave the disk unencrypted. Instead, the computer keeps the key in RAM and uses it to decrypt the accessed data blocks on the fly.
Look up Avogadro