Normally a release pin on a light cable attached to the pylon. As the bomb falls clear of the pylon the cable pulls the pin and enables the prop to spin freely.
The AA-11 has a similar light cable connected to the pylon too, which I guess has something to do with arming the warhead after launch.

(Saw a video of a guy fitting an R-73 to a pylon and the front of the pylon opened in two and the cable came out of there.)