What is the meaning of "the actual pillar is rendered unlawful" (yaḥrum ar-rukn al-fiʿlī) in the Shafi'i context when the congregant precedes his Imam by two pillars, one verbal and the other actual? And is it necessary to repeat it, or to repeat the entire raka'ah, or is the prayer valid?

The meaning of their saying, "the actual pillar is forbidden," is that it is forbidden for the congregant (ma'mum) to precede his imam in an actual pillar such as bowing (rukūʿ) or prostrating (sujūd). He incurs sin by doing so, even though his prayer remains valid. It is recommended for the congregant to return to the imam to perform the pillar behind him if he preceded him intentionally, or to wait for him. If he preceded him forgetfully, he is given the choice between returning and waiting. If the congregant precedes the imam by two pillars, his prayer becomes invalid, but if it is by less than two pillars, it does not become invalid. Preceding by a pillar intentionally is forbidden.