Basically, when you're annealing aluminium, what you don't want is a pool of molten metal on the floor - which is very easy to do as (very inconveniently) aluminium doesn't change it's colour as it's heated (unlike copper, brass, steel etc.) to give you an idea of the temperature you have reached.

The idea with the soap trick is to coat the washer (or whatever aluminium part you want to anneal) with soap, the old school hard stuff is best, from a block. Rub it all over the surfaces. What this does is allow you to get a visual tell-tale when the aluminium is up to the right annealing temperature as the soap will soot and turn black. At the point where the soap turns black, you're done. Remove the heat and quickly quench in cold water. You need to quench quickly as even after removing the heat, the temperature of the metal will continue to rise further for a short time.

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