Ishmam is another special manner of stopping. First the final consonant is pronounced with sukun and its sound is completed; then the lips are rounded in the shape used for dammah. The motion is visible, but no new audible dammah is pronounced.
The contrast with rawm is essential: rawm can be heard, because a reduced vowel is spoken; ishmam is seen, because only the mouth shape indicates the original vowel. If the reciter produces a distinct new u-sound during ishmam, the intended procedure has changed.
Ishmam is neither an ordinary final vowel nor an additional long vowel. It means fully stopping, then showing the previous dammah by lip position. The method is easier to grasp from an oral demonstration than from writing alone.