In «بِسْمِ اللَّهِ», the final ha carries kasrah. During continuous recitation, that kasrah is sounded; on stopping, the ordinary method makes the ha still. This matters because one reader may mistakenly preserve the whole final vowel while another may weaken or lose the final consonant itself.
Iskan practice: articulate the final ha clearly, then stop without pronouncing its kasrah.
Rawm practice: under oral guidance, produce only a faint fraction of the kasrah rather than its full vowel sound.
Ishmam is not used here, because the underlying ending has kasrah rather than dammah. The letters and proper respect for the divine name remain intact; only the permitted treatment of the final vowel differs.