What it does
Tracks three running counters about your usage.
How it works
Each shake increments today and all-time. A separate timestamp tracks when the current 'session' started; on a transition out of shaking, it computes the duration and updates the longest record if needed.
Where to find it
Settings → Stats section.
When to use it
Curiosity. The longest-session stat in particular is fun — it tells you the longest unbroken stretch you actually used Shake It On.