Pass-through a/v is one of those features that some customers may not be thinking about or even know about before they buy headrests DVD players, but it may be a feature they're looking for. If you plan on connecting an external video source such as in-dash DVD player or external game system and hope to watch your input signal on 2 or more headrest monitors at the same time, then "Pass-through a/v" is a feature you will want to have.
Pass-through a/v allows for you to input an a/v signal into the RCA a/v cables into headrest #1 and have the a/v signal "Pass-through" the a/v output on headrest #1 and into the a/v inputs on headrest #2. When you select a/v input on the headrests you'll see the a/v input on both headrests.
NOTE: with the a/v setup in a "chain" type of connection, if you wish to have headrest #2 see the external input video you'll also need to have headrest #1 also set to view the a/v input signal so it can "Pass-through".
If "Pass-through" is a feature you're looking for, take a look at the AUTOTAIN DREAM as one of the few models that does include the "Pass-through" feature.