If you don't catch him right away, he'll begin to develop his modus operandi, or MO, and probably get better at the crime. Maybe he'll find a more efficient way to kill someone or a quicker way to abduct a woman from a car. He'll start showing more control over the crime. While the MO tells us something about how he did it, the signature gives us some insight into why . Evidence of torture is a signature.
Signature is a way of linking cases.
Like a real signature, it's a personal detail that's unique to the individual. Sometimes there's a fine line between an MO and a signature. That's because the MO evolves, while the emotional reasoning that triggers the signature doesn't. The method a killer uses to get women into his van may change, but the fact that he always tortures them once they're inside stays the same. One subject covered the faces of his victims.