Osteoclastic activity refers to the body's process of breaking down bone in order to build it up again.
When consistent force is applied to a tooth, osteoclastic activity breaks down the bone in the jaw, allowing the tooth to move. At the same time, osteoblastic activity builds up new bone to fill in the space left behind the tooth.
Osteoclastic activity takes 48–72 hours to fully begin, but only takes about four hours to stop. This is why it's essential for patients to wear their aligners at least 22 hours per day. When the aligners go unworn for several hours, it can take several days for the activity to start up again.
The first few days of each step are particularly crucial, because aligners apply their maximum force when they are first worn. If your patient absolutely must remove aligners for an extended period of time, it's much better to do it at the end of a step than at the beginning.
Osteoblastic activity is slower than osteoclastic activity, which is why orthodontic results often take 6 months or more to stabilize.