German for "to put on top of" and refers to the brewing practice of putting one batch of wort "on top of" another.
In brewing, the newly oxygenated wort is added to fermenting wort at its peak fermentation. The oxygen in the fresh wort causes the yeast to revert back to its aerobic metabolism state from its current anaerobic metabolism state.
Breweries may use drauflassen to jumpstart yeast quantities when a small volume of new yeast is moved from propagation to fermentation vessels. Breweries may also use drauflassen when their fermenters are much larger than their kettles.