A three-axis accelerometer measures acceleration along X, Y and Z — forward, sideways and up. It is the smallest and least glamorous sensor in the ring, and it does more work than any of the others.
Movement as a sleep signal
Estimating sleep from movement is an old and well-validated idea: patterns of motion and stillness across a night separate sleep from wake with reasonable accuracy, long before any optical sensor is involved. The technique predates wearables by decades.
On its own it has a known weakness — lying still is not the same as sleeping. Combining movement with heart rate and its variability is what turns a rough estimate of time asleep into an estimate of the stages within it.
Effort, not steps
A step count is a poor description of a day. Ten thousand steps accumulated on a commute and ten thousand steps at pace cost the body very different amounts.
Aurora separates sustained, intense movement from ambient activity, so that a hard session and a long walk do not arrive in the morning as the same number. That distinction is the difference between a figure you act on and a figure you ignore.
What it cannot tell you
Accelerometry alone cannot see sleep stages, and it cannot tell what kind of exercise you are doing — only how much acceleration was involved and for how long. Sitting perfectly still while awake will read as quiet, and a restless sleeper will read as more awake than they were.
These are the limits of the method, not a defect in the device, and they are the reason no single signal on this site is read in isolation.