Imalah is a specific recitational inclination: the fathah shifts toward kasrah, and its related alif sound shifts toward ya, to the degree required by the transmitted reading. It is not permission to replace vowels at will.
A well-known occurrence in the discussed reading is «مَجْرَاهَا» (Surah Hud 11:41). The ra here is treated lightly, and the actual sound of imalah should be learned from a qualified reciter.
Imalah is neither an arbitrary mixture of two full vowel sounds nor an ordinary written kasrah. Its precise sound cannot be supplied by a verbal definition alone; oral demonstration and attentive imitation are essential.