Fish keeps Ray immobile, why?

I've seen this behavior a few times over 2 months of diving. I assume the fish has egg sacs nearby (in the sand?), but does anyone know for sure why, or maybe other theories for this behavior? I'm a relatively new diver and am just starting to get acquainted with marine life.

The fish isn't holding the ray down. It's shadow feeding. The ray digs up crustaceans and things from the sand, and the fish snaps up any scraps or things the ray flushes out, which don't see the fish hidden behind the ray. Sometimes the ray will take off for another spot and the jack will hitch a ride. Sometimes there's an interesting color change phenomenon too (though I don't see it in these pictures) where the jack will be black when it's shadowing the ray and will turn silver when it swims away from the ray to grab food, then turns black again when it returns to its ray. I've also seen a jack chase off other jacks that tried to horn in on "my ray"!
Nature, you couldn't make this stuff up!

So don't be confused trying to identify the "black" jacks that you will see with the rays. It took me a while until I saw one of the black ray jacks turn back to its normal silver.

drl got it. I've watched bar jacks shadow several species including hogfish and even morays. Here in my own waters, kelp bass do something somewhat like shadowing in that they follow another predator (morays) and snatch the poor blacksmith that escape from the moray's onslaught. Not quite the same as true shadowing where a predator generally follows a less-threatening species as viewed by its potential prey.