Explanation/Reference:
Explanation: When WPA-PSK or WPA2-PSK is performed via Temporal Key Integrity Protocol (TKIP) or Advanced Encryption Standard (AES) for the encryption, the client must go through the process known as the WPA 4-Way handshake for both the initial association and also when roaming. As previously explained, this is basically the key management process used in order for WPA/WPA2 to derive the encryption keys. However, when PSK is performed, it is also used in order to verify that the client has a valid Pre-Shared Key to join the WLAN.
Reference: http://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/116493- technote-technology-00.html#anc6