separate key-frame animations?

Hi!

I added a mask to mask a portion of the video at the beginning of the clip. After 2 seconds or so i don't need it anymore until the last 2 seconds of the clip. How do i remove it until when i need it again at the end?
I tried all sorts of "Opacity"/"Expansion" animations with the mask, but it just doesn't work. This is another problem: how can i have 2 separate animations? for example, if i have 2 pairs of key-frames - i don't want any animation between one pair to the next, meaning - when time passes from the 2nd key-frame to the 3rd - i don't want any animation to happen between them, even if i have several options with the stopwatch clicked.