that rollover effect with diagnol masking?

hey guys, goto the site, put in ur birthdate, click enter, and then click on “ONLINE MIXER”.

I’m trying to recreate that effect where you roll over one of the menu options(HIP HOP) but i’m having trouble figuring out how that white glow diagonally goes across the choice frame.

I know i could just put a white sphere with a little alpha in the corner and just let it grow, but i dont think that’s how it’s done or if that’s efficient because the sphere then is much larger than the area just so i can cover it.

let me know what u guys think.