I think these are attached to the tray table mounting, which is on the fixed part of the seat in front (armrest assembly). The seat reclines independently of this. I was looking at one at the weekend actually on a J flight with Latam on an ancient 777 where the leg rest was broken, and another passenger was trying to use hers.

It didn't look very comfortable, even with a lot of leg room. I'd imagine it would affect the passenger in front to a degree.