It seems to be just previously, it does:

  switch (op) {
    case "call_function":
      interpret(op.function)
    ...
  }

Now it does:

  switch (op) {
    case "call_function":
      ... setup frame objects etc ...
      pc = op.function
      continue
    ....
  }

Not sure if they just "inlined" it or use tail-call elimination trick.