What does “lā maḥalla lahā min al-iʿrāb” mean for an inserted clause?

Iʿtirāḍ and the Relative Measure of Ījāz and Iṭnāb — Concluding Volume 1

• Its words lack internal syntax
• It occupies no syntactic position in the host sentence
• It is always an imperfect verb
• It must be an accusative noun