Bâtonnage is the French term for stirring the settled lees (expended yeast) back into the wine.
Bâtonnage, or lees stirring redistributes polysaccharides and mannoproteins of dead cells into the wine mass and, in the case of barrel aging, re-exposes the wine to the wood at the bottom of the barrel. Lees absorb oxygen, in the absence of which the wine will become reductive. Bâtonnage tends to enhance the characteristics of sur lie aging, diminishing fruitiness and slightly reducing wood/oak influence.
Source: Ben Rotter http://www.brsquared.org/wine/Articles/surlie/surlie.htm