I just noticed this. When I scrub or drag the CTI along the timeline where there is the red line going across indicating unrendered video or stills, what I am seeing displayed in the timeline monitor are fully rendered frames with all the effects applied. I do not see all the frames in the sequence being displayed, just the ones that PE has time to fully render. This is different than using the shuttle below the monitor. The shuttle tries to display all the frames in the sequence while sacrificing quality.
You've probably noticed how fuzzy stills look if you don't render them first when you play them back on the timeline. But if you slowly drag the CTI across the timeline, you'll see fully rendered frames displayed, but only the ones that PE has time to fully render. You can pile up the effects on a clip, and when you drag the CTI, you'll only see frames that have been fully rendered.
I have noticed this "feature* too, Robert. When I am trying different values for an effect on a very long clip, instead of rendering the preview each time, I just "play" the video using the forward arrow key and the preview looks good.