Spin off a volley is a common symptom of a double contact, but it is not a guarantee.
Quite a controversial and contested topic in beach volleyball is when a double or lift/catch is called during a volley or set. The rules being infringed in these two possible ball faults are:
9.3.3 CATCH: the ball is caught and/or thrown; it does not rebound from the hit. (Exceptions 22.214.171.124, 126.96.36.199).
9.3.4 DOUBLE CONTACT: a player hits the ball twice in succession or the ball contacts various parts of his/her body in succession.
In plain English this means that you cannot catch or hold onto a ball for too long, and you cannot touch a ball twice in succession. Due to the nature of a two handed set or volley it is very easy to either be blown up for catching the ball or for touching the ball twice during the volley, also known as a double.
Many people use a spinning ball as the result of a volley as an indicator of a double touch. Their reasoning says that the only way to create spin on the ball with two hands is to touch it for two different durations during the release which results in the spin.
Referee John King breaks down why this isn’t not necessarily the case and what beach volleyball referees look for during volleys or sets. He also covers whether it is allowed to receive a hard driven ball with two separated open palm hands during play.
Fault for double contact: As explained in the video spin is a common symptom of double contacts, but not a guarantee. You can be faulted for double contact while setting as you receive the ball, yet still release a clean no-spin perfect ball. This is still considered a double contact fault. Tt’s not just the release but also the initial first contact with each hand that counts. (a lot of people refer to this as a “catch”). *
The exception to these rules is during a hard driven ball. Then the ball contact can be extended momentarily even if an overhand finger action is used. Referee John King refers to this as well.
This can be found in the official FIVB rules – Point 188.8.131.52
*This video and explanation was found in this reddit thread: https://www.reddit.com/r/volleyball/comments/3kznt5/can_someone_help_me_find_the_rule_about_spin_on/