The text is created in Premier Pro titler

Video track1: text
Video track2: bagground video
Video track3: bagground text video

The video for track 2 and 3 is the same.
On the video for track 2 I have put in a fade-out key on opacity.
On the video for track 3 I have put the "Track Matte Key" found in "Video effects/Keying", there I sat the Matte option to "Video 1"
So now when the surrounding video is fading out, the track 3 video will show up in the text.

I have done nothing special about the text other than altering the tracking between the text so it will get a more square shaped text box-look.

I hope this helps you, if not I will try making a graphic view of it.