Why this term exists
Readiness is often confused with maximum activation. Tonus separates useful preparedness from both slackness and over-tension.
Definition in the system
Optimal Tonus is adaptive. A system can be ready in more than one configuration, and the best state shifts when the task, environment or expected demand changes.
Tension is only one variable that can tune Tonus. More tension can improve readiness up to a point, then reduce flexibility, accuracy or efficiency. The useful region is therefore a working zone rather than a universal maximum.
Tonus pairs naturally with Takt: Tonus asks whether the system is ready; Takt asks whether the timing and phase relations allow that readiness to become an effective event.
Example
A violin string is useful neither slack nor tightened to breaking. It is tuned for the note and function required. Change the tuning target and the optimal tension changes too.