"Ball Return" mechanism... what is it?

I have a question,

what is this “Ball Return” mechanism that is described in section 2.3 of the rules (under Playing Field Construction)?

Where is it located?
How high is it?

It has been removed, so you don’t have to worry about it.

It was basically like a “doggy door” on a garage. An opening about 12 in x 12 in at the bottom of the driver’s station so the balls can be passed through it. FIRST has decided to eliminate it from the playing field (see the first team update).