Water is brought to a boil with a few tablespoons of regular vinegar. The vinegar causes the egg albumen to tighten into itself. Egg white tends to spread throughout the water without the vinegar.
The water is off the heat. No longer boiling. At sea level that is 10 degrees Fahrenheit higher than where I am at in Denver. But not to worry.
Egg whites denature between 144-149℉ 62-65℃
Egg yolks denature between 149-158℉ 65-70℃
And that is much lower than boiling in any case.
Water can be lifted to the top of the egg to distribute the heat.
The yolk is lifted out of the water. Surplus egg white globs over the edge of the spoon.
The edge of the spoon is used to trim the egg white by scrapping it against the side of the pot or trimming with a knife. The size of the spoon determines the amount of egg white retained.
Egg white left in the pot. Cleared out. The water is used again to reheat the poached egg later.
The poached egg is transferred to cold water which halts cooking action. The egg can be held for hours then reheated in the same water that cooked it.
The whole point is to deliver a bladder of egg yolk sauce to an awaiting pile of starch or protein or vegetable, in this case leftover macaroni and cheese with crab.
You can do the same thing with a fried egg.