There are 2 forms of attenuation for beer. The first is the limit of attenuation and it is set by the wort composition and the species of yeast (s. uvarum (lager) vs. s. cerveciae (ale) or however you spell this) but not the strain of yeast. This limit of attenuation indicates how much of the extract (which is sugars, proteins and compounds other than water) can be fermented by the yeast. The limit of attenuation can be determined with a forced ferment test where a large amount of yeast is pitched, kept in suspension (shaking or stir plate) and fermented warm.
The actual attenuation depends on the strain, health, fermentation conditions, temperature ..... The more flocculant the yeast, the bigger the difference between actual and limit of attenuation will be because the yeast drops out before the job is done. The lack of significant circulation also helps dropping out the yeast.
The bigger the difference between actual and limit of attenuation, the sweeter the beer will be and the smaller the difference the drier it will be. If you want to get closer to the limit of attenuation you can:
- pitch more but you may get the problems associated with over pitching
- rouse the yeast toward the end of fermentation
- pitch a less flocculant yeast at the end
The latter is best done with a large starter (keep some wort from brew day) since there won't be much yeast growth in the beer due to the lack of nutrients (amino acids, O2, Zink ...) Don't worry about getting much flavor from the second yeast since the main profile has already been set by the 1st yeast.
German Lager brewers may do this by adding some less flocculant yeast when racking the beer into the lagering vessels to make sure that they get the a dry finish in the beer if they haven't fermented with a "dusty" yeast in the first place.
Kai