So, for those who don't know what a cold break is (apart from a holiday in Murmansk) I suppose I'd better tell you.... it's a bit technical so just switch over now if that's put you off!
When the wort is boiled with the hops we add a set amount of "protofloc" tablets, which are ground up seaweed. The idea behind yet another aquatic process aid to beer is that the gloop contained therein causes proteins in the wort - which need to be limited to produce clear beer - to clump together (much the same way sturgeon swim bladder flocculates yeast into clumps) and become trapped in the hop bed at the bottom of the copper when the wort is run off to the fermenter. Rapidly cooling the wort through a plate chiller makes this process happen a lot quicker and more efficiently and is pretty much essential to create clear beer in the end! Not all the break material gets trapped and some gets through, but this then flocculates in the fermenter and all is well...
The specific gravity test is done before the yeast is added and gives you an exact reading of the sugars in the wort you are about to ferment and, thus, when a reading is taken before the beer runs off to conditioning tank, with the aid of a clever spreadsheet you can work out the exact alcohol by volume of the beer; simple, eh?
Never one to do one thing when I can do two at the same time and thus expend less energy I could then use pissing about on the internet, I leave the wort in the sample tube to ensure the "cold break" has occurred properly... which, as you can see, in this case it has!