Write each letter's name in full and add the letters of that name. י = יוד = 10+6+4 = 20; ה = הי = 5+10 = 15; ו = ויו = 6+10+6 = 22. Filling יהוה this way gives 20+15+22+15 = 72. Subtracting the letter itself leaves the 'hidden' value — aleph: 111 − 1 = 110 — which the handlists call mispar ne'elam.
Meaning
The fill is the cipher of the divine names, and its famous product is the four ways of spelling out the Tetragrammaton — 72, 63, 45 and 52 — which the Zohar reads as the four worlds and the four Partzufim. It also makes the letters generative: aleph's own name is worth 111, so the silent letter is said to contain a word of its own.