It has three sensors that notice if it is changing speed up/down, left/right or back/forth.
A step will result in a speed increase up (foot go from still to up), followed by a sharp speed increase down (foot go from up to down), then “up” again (foot go from down to stop).
Going up a stair causes the same but different timing between the speed changes.
If you are on Android, the Physics Toolbox Suite can let you look at the exact values it is measuring.