When adding a title to an iMovie project, you typically drag a title style from the Titles browser and drop it onto the beginning, end, or middle of a clip (the middle gives the title the same duration as the clip). If you have a very long clip, your title could end up being much longer than you wanted.
Instead, hold the Shift key as you drag. A four-second blue highlight indicates where the title will end up when you let go of the mouse button. You can skim across your footage and select the precise spot where you want the title to begin. And because it's four seconds, you probably won't need to edit it (unless you create a lengthy title that takes longer to read).
Take a look: