Actually, Tang Feng also had a sudden inspiration.
Due to his expertise in medical skill, he was very familiar with the human body's meridians.
The human body's meridians are divided into twelve meridians, the eight extraordinary meridians, and the central meridian as described in Vajrayana Buddhism.
If the meridians are clear, qi and blood flow freely, and no illnesses will arise.
If the meridians are blocked, qi and blood become obstructed, and illnesses will arise in abundance.
He had previously experimented with silver needles on himself, finding that when some acupoints were needled to cause qi and blood to flow in reverse, various discomforts and even extreme pain could occur in the body.
Especially some acupoints, a single prick could bring about unbearable pain.
Using this method for interrogation might achieve unexpected effects.