Which restriction distinguishes this defined iʿtirāḍ from iḥtirās?

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

• A purpose other than merely averting an implication
• That it be a past-tense verb
• Only through a preposition
• That it must never follow discourse