In article
, Farm1
writes

I've always used half a dozen tennis balls when I dry my duvets but it
makes a hell of a racket as the drier tumbles. The tennis balls keep
the drying feathers moving. They break up clumps of drying feathers
and help the feathers dry evenly.



Likewise for 'stuffed' jackets - Puffa jackets, etc.

--
regards andyw