I'm quite tired at the moment, but hopefully this will help :)

In the help 'contents and index', do a search on 'applying video fx', 'keyframes' and 'split'. This should give you a fairly good background for what you are doing.

Then try the following:

select the video event (single click on it), position the cursor and split it at the point you want the fx to start from and at the point you want the fx to stop.

You will end up with three events. Add the fx to the event you want it on only.

NOTE: depending on your preferences, you may get a small fade appearing at the split point. You can manually drag this fade to get rid of it, or uncheck the 'fade edges of video event' box in the Preferences>Edit box. (this will effect subsequent edits, not existing ones).

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Alternatively, using keyframes to adjust the effect from being effectively on (ie with parameters adjusted to give an effect) and off (choose the 'reset to none' preset or equivilant) at the right point in the event.

Again, reading up on keyframes should help here.