Do this
Take the pan off the heat. Wait ten seconds. Add cold butter one small piece at a time. Swirl the pan instead of stirring.
Why it works
Cold fat melts slowly and breaks into tiny drops. Those drops spread through the liquid and scatter light. That is the shine.
When to skip it
Skip it for a sauce you will reheat. The emulsion breaks on the second heating and the fat floats to the top.
