this would probably be alot easier in ae, but if you want to do it in premiere there are several different ways. if you split up word by word, then you can control each word with fade in and fade out. if you want the text to remain, but just darker, you can double up the titles on two tracks. one with the entire sentence, the other with the words split. if you want to use a single title, the brightness & contrast effect can be used in combination with its mask. using keyframe animation to control the brightness and the position of the mask. there is also a lighting effect, which you can animate to move over the sentence like a flashlight highlighting words as it moves across them.
1. Create a crawling title of the words.
2 Create and place Adjustment Layer (AL) in a track above it.
3. Add a lighting effect to the AL. The light effect has four lights. Activate three of them as OMNI. Place one in the center over the words, adjust it's radius to one word with. Place the other two OMNI's right and left of center. Give the right and left OMNIs a negative intensity and adjust their radius to create a ramp effect (fade to black) on edges.
4. Stretch the crawling title until it's pace matches the VO track.
Thanks for all the replies so far... just to clarify what I'm envisioning, I'm thinking of a more artistic placement of a word that quickly fades up to a brightness that makes it impossible to read because it "flares' outside the normal bounds of the lettering, and then quickly fades down. I'm sorry I don't have an example to show... I can only find the lens flare "side-to-side" flashlight effect described by some other people. It's possible that this only exists in my head, but if anyone has seen this effect before please let me know. I think the earlier responder is right (that AE would be better for this), but I don't have much experience with that app yet.
Thanks for the help so far.
You could add some Fast Blur or Camera Blur in conjunction with "Brightness" to create a "blooming" effect that will enhance the "unreadability" of the words at the time.
Key frames are your friends for this in PPro or AEFX.