Extending or freezing a frame allows you to pace your video better. More importantly, since freeze frames are applied on a per track basis, it buys you more time for something like a voiceover or an annotation.
Freezing the frame
Move the play head to the frame you want to extend. Right-Click on the track you want to extend and choose the Extend Frame option or press SHIFT+E. For the keyboard shortcut to work, the track must be selected first.
Enter the desired duration in seconds.
This creates a stitched clip, provided that you have not disabled auto-stitch in preferences.
If you have auto-stitch disabled, the clip will be split in three as shown below, with the middle clip containing the extended frame.
In order to change the duration of the freeze right-click on the still portion of the clip and choose the Duration option. This works with both stitched segments and split segments.
Removing the freeze
Now let us look at how to remove the extended frame since it is not very straightforward. Extending frame always involve splitting the clip. By default, this is followed by a stitch. Camtasia has an unstitch option but does not have a dedicated undo split option. Once you extend the frame, the only way to undo the extend operation completely is the general purpose Edit ➾ Undo.
If the general undo operation is no longer feasible/available, you can still remove the extended frame by either setting the duration to 0 or by an unstitch followed by a delete clip. This would, however, leave unnecessary splits or stitches in the clip.