One that learns your own pattern from what you log, instead of applying a table built from other people.
Bodies do not agree with each other. A signal that means back off for one person barely registers for the next, and cycle phase is a clear example: the average difference across a group is smaller than one person's own swing from day to day. A table that says do less this week because of the date is therefore wrong for a large number of the people it is applied to, and there is no way to tell in advance which ones.
Your working weights come from the sets you have logged, not from a phase. Your recovery is measured against your own baseline rather than an average person's, and in the luteal phase against your own history from that same part of your cycle. If you log a symptom — cramps, low energy, poor sleep — the day's suggestion can ease off. It can never harden. A symptom you report is allowed to make a day gentler and is never allowed to make it heavier.
It rules out telling you what your body is doing when we cannot know. Rhythm suggests; it does not diagnose, and it does not demand. And an easier day is never an empty one — there is always something named that you can still do.
Cycle tracking is optional and open to anyone, whatever your gender. Your cycle dates and phase are worked out on your phone and never sent to our servers.