Freeze must be called after all key frames have been added (see method AddKeyFrame(Int32, TimeSpan, SrtTransform)) and before the animation can be used in the animation system. Freeze optimizes the internal data for fast access at runtime.

When key frames are added or removed after calling Freeze, the animation is automatically reset into an editable state. All internal optimizations will be discarded! Freeze needs to be called again after all key frames are added/removed.

Weights can be modified before and after Freeze. For example, weights can be modified while the animation is running.

Reference