The last pass stops answering and starts writing instructions. The instructions are a shape too, and the shape is the reason a compiler is a compiler rather than an interpreter.
About 1 min of reading
Write instructions and prove agreement
Write three instructions
Before you run it, say out loud what it will print. Then open the answer.
Run the machine
Before you run it, say out loud what it will print. Then open the answer.
Prove the answers agree
Before you run it, say out loud what it will print. Then open the answer.
A small set is a machine that is easy to write. The machine is written once and the compiler is written many times, so the set is small.
A slot has a number. A number is a place, and a place is what makes a value live rather than copied.
It says what to do with which slots. Slots rather than values is what makes the instruction runnable later.
The children are written before the parent. The order is the same order the evaluator used, and the sameness is the claim.
The evaluator answer equals the run answer. The equality is checked by a test, and the test is the whole proof.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.