Rippling is a type of editing that moves all clips along if you insert another clip before it in the timeline. This also means that if you trim a clip, all affected clips will move along to fill the space created. By default, iMovie ripples clips.

To temporarily stop rippling from happening, hold down the Command key while doing an edit. This will stop all other clips from moving, so any gap you create in the timeline will appear as blank space.

If you want to extend a clip and trim the next one along, you can do this in one process by disabling rippling. Hold down the Command key and drag one clip over the next one along. Normally this would push the next clip along, but with rippling disabled, it overwrites that portion of the underlying clip.