When creating a keyframe animation, you might want to change the visibility of the parts for different keyframe - for example, you might want to show all your models during a fly-around, and then focus on only some of the parts. There are two possible ways to achieve this.
Insert commands into the keyframe
Create your entire keyframe animation, ignoring for the moment the parts visibility. Once all your keyframes are made, open the keyframe edit window at the Speed and Action section. Set the For keyframe field to be the keyframe at which you want to change the visibility attributes. Now, enter in the Commands to execute at keyframe box the commands corresponding to hiding / showing the parts that you want to execute. Keep in mind that the length of this box only allows you to have 10 commands line, which could not be enough. In this case, you can work-around the issue by writing your commands in an external .enc file and call it from the Commands to execute at keyframe box with the play: command.
So the keyframe edit window would look like:
and the visibility.enc file would be something like:
VERSION 10.0000 (a)
1 2 3 4
part: visible OFF
part: visible ON
An alternative method to create an animation where the visibility of the parts changes requires using EnVe. Create a first keyframe animation from the beginning to the keyframe where the visibility changes. Save this first animation. Then, create a second keyframe animation for the part of the movie you want to create with different parts visibility, and save it as a separate animation. Now you have two animation files that contain the first and second part of your movie. Simply use EnVe to merge them into a single animation.