I tried it with the X-meter bracelet. As long as the frequency is appropriate and the amplitude is large enough, it will trigger step counting regardless of the posture. The patent description of the FXXXit bracelet uses FFT to identify the frequency, but looking at its advertisements, it seems that some kind of threshold judgment method is used, possibly to achieve a balance between power consumption and accuracy. So for a wristband, the reason why it is tied to a pendulum and does not count steps may be because the acceleration is not enough.