for info from wikipedia...
During the double dropping process the wort (newly brewed, fermenting beer) is first fermented for a period of time before being transferred, under gravity or by other means, into a lower vessel where it continues fermentation. The dropping process has two primary effects on the beer being fermented: the trub that has settled during the first period of fermentation will be left behind, leaving a cleaner beer and a cleaner yeast to crop from the beer for the next fermentation; the second effect is the aeration of the wort, which results in healthy clean yeast growth, and in certain circumstances butterscotch flavours from the production of diacetyl.[16][17]
Breweries using the double dropping process include Wychwood Brewery who contract brew Brakspear branded beers,[16] and Flack Manor.[18] Marston's use the name Double Drop for one of their beers as they use the related brewing method of the Burton Union system.[19] Wychwood transfers the wort the morning after the day fermentation started - typically about 16 hours later. This process originally took place at the Brakspear brewery in Henley. When Brakspear moved to the Refresh UK's brewery in Witney, a new brewery was built to include the original double dropping system.[20] Brakspear claim that some of the flavour common to its beer is due to a combination of its very old complex multi-strain yeast and the dropping method which encourages it to produce the butterscotch-flavoured compound diacetyl.
