A fluid or hydraulic coupling is a device used to transmit rotational power from one shaft to another. It consists of two halves, each of which contains a set of vanes that are free to rotate in the housing. When one shaft is turned, the fluid inside the coupling is forced to flow through the vanes. This creates a torque on the other shaft, which causes it to turn.

https://medium.com/@prajwals1804/6-main-benefits-of-fluid-or-hydraulic-coupling-357e2a5e21f3