Taʿdīl al-arkān denotes the required composure and steadiness in prayer's bowing, prostration, and related postures. The disagreement asks whether omission compromises a necessary quality or the essential completion of the pillar itself.
In the positions attributed to Abū Ḥanīfah and Muḥammad, the original act of prayer is discussed with a possible remedy for the shortcoming. Abū Yūsuf and al-Shāfiʿī assess this deficiency more strictly.
Usūl point: The same external omission may be classified either as a defective attribute or as failure to fulfill a pillar, producing different rulings on validity and repetition.