Four letters stand between a lot of people and a very large amount of money.
QDRO. Pronounced "quadro" by everyone who works with them, and pronounced nothing at all by everyone else, because most people have never heard of it until an attorney mentions it in passing.
It is a court order. It is also the only legal key that opens a 401(k) to anyone other than the person whose name is on it.
No key, no money. It does not matter what the divorce decree says. It does not matter what anyone promised. Federal law built a wall around retirement plans, and the QDRO is the only authorized door.
Here is how the door works.