What is speed-rule in volleyball?

Answer 1

Could be referring to the time limit (ten seconds) the server has to serve the ball once the whistle has blown. Could also refer to the rule changes that have been implemented to speed up the game, such as scoring on every serve regardless of which team serves.

Answer 2

In volleyball the speed of serve is measured. Speed is measured with radar gun. The best players can serve about 120 - 130km/h.