This is the first of the technical posts I warned you about. Depending on the depth of your geekery you will either stop reading very shortly indeed, or lap up the wonderful information I am about to pour forth with.
I read up on flocculation recently in order to write an assignment for my brewing course and learned a number of interesting things in the process. I knew little of the mechanism of yeast flocculation, and was interested to learn that it appears to one of those things that is of little use to the yeast themselves, but crucial to the production of the beer we all love so much. First of all, flocculation was mistakenly attributed to individual yeast cells merely falling from suspension under the action of gravity and collecting in a smelly mass at the bottom of the fermenter. Studies have shown that flocculation actually involves complex interaction between groups of cells that clump together at very specific times in the fermentation cycle. Usually this is during the stationary phase of yeast growth when they have carried out the sterling work of transforming the sugars into ethanol.
In order to understand why the yeast decide to flocculate during this phase of fermentation we must turn to the yeast cell wall and look at the manner in which the cells stick together. The current thinking in yeast studies refers to the 'lectin theory of flocculation'. Lectins are long chain like molecules that stick out from the cell and bind to receptors on neighbouring cells. The receptors are also binding points for sugars, which wort is awash with. This is why most yeast strains will not flocculate before all the sugars are gone; the receptors are tied up with sugars and the long lectin molecules cannot gain access and hook up with the neighbouring cell. From a practical point of view this might explain why that beer you want for the stag night next weekend is refusing to clear; there is just too much priming sugar floating around and the yeast hasn't managed to get through it all yet.
Further practical issues with flocculation involve either premature flocculation, or a stubborn refusal to flocculate at all. Flocculation too early in the fermentation process results in under attenuation of the beer, with resultant alteration to expected character through excess residual extract and drop in ABV. Refusal to flocculate causes over attenuation, and a hazy beer if you're a home brewer, or a massive load on the filtration system if you are a commercial brewer.
There are a number of ways to reduce possible problems with flocculation, the most obvious of which is to avoid those strains of yeast that are known to hang about longer in the beer than is perhaps useful, or drop like a stone at the first opportunity. Certain beer styles require very specific flocculation characteristics to achieve distinctive flavour and body. The strain used by Fuller's for their ESB comes to mind because it flocculates very readily, and this gives the familiar full body caused by high residual extract.
A further way to maintain expected flocculation patterns is to ensure there is sufficient calcium present in the wort. Calcium is absolutely critical for all flocculation as well as mashing and break production, so add it to the brewing liquor if it's in short supply.