Dynamic RAM Preview seems to work quirky for me (quirkily?).
I build a little project with a chroma key, bkg, and titles. I place a region area and Shift+B. The area appears to render and waits for me to press the Space Bar. I do and it plays the same speed as before the render - sometimes. Other times, it works just fine. And I can't tell what I'm doing differently to make it work on those Othertimes.
And Sometimes, I'll place region around a hard cut and (from main event on track 3, to cutaway on track 2 back to main event) and it will build only to the cutaway.
I can typically get about 7-8 seconds of RAM Preview, and that last example only uses about 5 (1 sec before cutaway, 3 sec cutaway and 1 sec after).
I've Cleaned Up All Prerendered Video and that doesn't change things.
Any suggestions? -mjr