If a word is already originally still at its end, its final consonant remains still when stopped upon. But when a word ends with fathah, kasrah or dammah, the basic method is to suppress the final vowel and make the letter still. This is iskan.
«نَسْتَعِينُ» — the final nun is vowelled with dammah in connection, still in the ordinary stop.
«الْعَالَمِينَ» — the final nun bears kasrah when joined and is still in the ordinary stop.
Iskan preserves the whole word while leaving its last vowel unspoken. It does not mean deleting the final consonant or adding a vowel to the letter before it. Rawm and ishmam are specific permitted alternatives to this ordinary way of stopping.