New keyframes are created by clicking Capture Keyframe 
on the Animator toolbar. Every time you click this button, Autodesk Navisworks adds a keyframe of the currently selected animation set, camera, or section plane set at the current position of the black time slider.
Conceptually, keyframes represent relative translations, rotations and scaling operations from the previous keyframe or, in the case of the first keyframe, the model’s starting position.
Keyframes are relative to each other and to the model’s starting position. This means that if an object is moved in the scene (when, for example, a new version of the model is opened, or if movement tools are used in Autodesk Navisworks), the animation is done relative to the new starting location rather the animation’s original start position.
The translation, scaling and rotation operations are cumulative. This means if a particular object is in two animation sets at the same time, both sets of operations are carried out. So if both are translating across the X axis, for example, the object will move twice as far.
If there is no keyframe at the start of the timeline for an animation set, camera, or section plane set, then the very start of the timeline acts like a hidden keyframe. So, for example, if you have a keyframe a few seconds in, and the frame has the Interpolate option enabled, then over those first few seconds objects would interpolate between their default starting position and those defined in the first keyframe.