This is not PFS but a simplified example of how you may not be able to decrypt a conversation with one side's private key:

Server X and User Y are communicating. Some guy Z has X's private key. Z is also passively listening the communication.

- Y sends its public key to X. X sends its public key.

- Y generates a random number (A), encrypts with X's public key sends to X.

- Both X and Z decrypt the number. Now X, Y and Z all know the number A.

- X generates a random number (B), encrypts with Y's public key, sends to Y.

- Since Z does not have Y's private key, it can't decrypt it . At this point, X and Y know A and B, Z only knows A.

- X and Y use a predetermined algorithm using A and B as inputs to generate a new key. Further communications are encrypted/decrypted with this key.

- Z can't decrypt the communication.