From Distillers Wiki
A thumper, or doubler, is sometimes used to increase the purity of distillate without having to redistill. The vapour from the still is directed into a jar (or some other container) of liquid, into which it condenses. As more vapour comes into the thumper, the temperature increases and the liquid evaporates again, effectively redistilling it. It is then condensed by the usual method.
Similar to the thumper is the slobber box, which contains no liquid and simply collects solids and condensate (called puke by old-timers) which have been brought up with the vapour. A slobber box can help reduce the amount of congeners produced by a relatively inefficient still.