When fast SSID changing is enabled, the controller allows clients to move faster between SSIDs. When the client sends a new association for a different SSID, the client entry in the controller connection table is cleared before the client is added to the new SSID. This happens when fast SSID is disabled.
With fast SSID enabled, the client entry is not cleared and the delay is not enforced .When fast SSID changing is disabled, the controller enforces a delay before clients are allowed to move to a new SSID.