Blu-ray can do subtitles two ways - using a system where the subtitles are rendered fonts (not used often) or where they are graphics, like in DVD - only with much higher bandwith and more flexibility in where you place graphics, and support for things like transition effects. These graphical "subtitle" streams are actually known as "Presentation Graphics" or PG streams. They can be used with any still graphics that fit within the bandwidth constraints, and are able to show video still images like with the two commentaries in Cars. This is part of Profile 1.0 and works on any player.
RV uses the same mechanism to do the "coach's clicker" style highlight effect on screen when the director's commentary is running on that disc.