Deauthentication Attacks are a type of denial-of-service attack that send a deauthentication frame to the access point. The attacker can input the MAC address of a client on the wireless network to deauth the client and force them to connect to the attackers evil twin access point.

To send a deauthentication packet there is no need for encryption, all the attacker needs to know is the MAC address of the victim, which can be sniffed in the clear.