Hope this is ok the link to YouTube. But here goes.
Taken from Wikipedia.
The resonant body of the tongue pipes has an apple-like shape on the apple shelf, which gives this musical instrument its name. 
In his work Syntagma musicum, Michael Praetorius describes the structure:
"Apffel or Knopff Regal is 8th foot Thon; Is halber its proportion / that it stands like an apffel uffm stalk / so gennet; The largest corpus is about 4 inches high / has a small tube / on the size like its mouthpiece / and uff of the same tubes a round pick button full of small holes / like a bendbutton drilled / since the sonus has to go out again: Is also after shelf kind of sweeter and much quieter / because to listen to another shelf / dienet wol in positiffen / so in chambers
The apple-shaped resonant bodies reduce the volume of the comparatively loud shelves and the sound becomes softer. In addition, the extraordinary look attracts attention.  Like all shelves of this era, the apple shelf is built in a choric way. The key circumference (gothic) is F,G,A,B-g''a''. The soundscape is related to that of trombones, bassoons and sods. 
The organist Paul Hofhaimer played on an apple rack in 1506 in a mass in front of Emperor Maximilian I.  This is recorded in the wooden engraving of Emperor Maximilian, the Mass by Hans Weiditz from 1518. Maximilian had this instrument built for his court organist Hofhaimer. Johann Gottlob Töpfer describes this apple rule in his work The Theory and Practice of Organ Building:
"One sees a the keyboard, b the wind drawer and c the bellows, which is lifted from a crutch d and is weighed by a dolphin-like weight e. The shelf has four rows of tongue pipes, which, however, seem to form only one register. The sound bodies, apparently cast out of brass, have a whimsical shape, which they form spherical and provide with incisions reminiscent of those of the bells. It is a so-called knöpflin-Shelf. The apple shelf is similarly built, but the spherical bodies have several sound holes."
To me it sounds like sackbuts, serpents and any other medieval wind instrument all in one place.