That's a good question

Dear Zhuo Chen,

Here is part of text where we have the explanation of your question “A single motor at PER’s core drives its legs in an elliptical or oval motion that makes for inherent stability, so it avoids falling forward or backward. Torsion springs generate added power in PER’s legs, making it still more steady. The paragon of dynamic geometry is unencumbered by any power-hungry, number-crunching processor that gauges steps in line with sensor data.”

For more information about this subject please take a look at links and attached file on topic.

https://www.popularmechanics.com/technology/robots/a29056447/super-robots-faster-than-humans/

https://www.newsweek.com/topic/robots?page=4

https://dl.acm.org/doi/pdf/10.5555/3281667.3281743

https://hackaday.com/tag/torsion/

Best regards

Dear Zhuo,

Here you have a good reference

Bipedal robot that runs like an ostrich can balance itself | Daily ...

www.dailymail.co.uk › sciencetech › article-4468748

May 3, 2017 - The bizarre bipedal robot that moves like an OSTRICH, can balance itself and ... Dubbed the Planar Elliptical Runner, the tiny bot can reach speeds of ... at the wheel: 'Faraday cage' principle is used by Nissan to block signals.