The ancients often used tortoise shells for divination. How do they divine?

First of all, the things to be divined are carved in the middle of the tortoise shell in ancient Oracle Bone Inscriptions (abbreviated as far as possible, within three words is appropriate), then burned on the fire, and the tortoise shell is taken out after it is cracked. If the crack doesn't pass through those words, it means a good sign. If the crack stops on the word, it means it is general. If the crack passes through those words, it means a bad omen.