These are two reasonable guesses as to why some demons freeze when you see them, as well as why most demons remain still when blended or otherwise making an appearance:
- You're not susceptible to certain acts of magic when the requisite kinetic energy (from motion) is not present, in that shadow energy can be cast, but will not affect the target unless and until the body is sufficiently tense from muscle exertion resulting from the initiation of a new motion, such as standing up, in combination with eye movement. The target's awareness of and focus on the caster enhances the results greatly; and,
- persons with magical abilities, whether known or unknown, that runs counter to other, demonic forms of magic can intentionally or inadvertently (and, even unknowingly) freeze a demon temporarily with one hand, while blasting them with another. Results vary depending on the variety of demon, as some are more sensitive to than others to blasting, and some are immune to freezing.