Hey guys I am an intermediate blender user and I want to do do the translucent animation on the foreground with
another animation in the background like the blender team did on Sintel between time frame 6:13 to 6:18 ??? Thanks in advance.
Sometimes. But in other cases (like when you’re using an alpha channel) you have to tell Blender how to composite things. Do this by playing around with Alpha Over/Under strips available on the Shift+A menu (select the strip you want to change first). If you see grey plaid, it will render as black because Blender is too stupid to realize that the one strip with alpha has data below it. Gotta love computers 
Also, if you’re doing it by animating the opacity, be sure to select Gamma Cross in the Blend menu as that’s what’s in style these days.