Group Key Handshake is a two way handshake that happens when an access point needs to deliver a new GTK to all client stations. GTK is used to encrypt the broadcast and multicast traffic. An important thing to remember here is that this is a separate process as opposed to PTK generation, if it was not a separate process, each time a new GTK was generated new PTK would have been generated as well. This would have created severe overhead on the WLAN since PTK is unique for each station.
NOTE: All clients stations must have completed an initial Four way handshake for the 2 way process to occur.
