Provoost's baby Jesus with a propeller was not exceptional. Around 1500, there was a veritable explosion of such images, most commonly with horizontal propellers (whirligigs). These three images give an idea how the toy was played
Christ Child with a Walking Frame, by Bosch (1480-1500?) http://www.levity.com/alchemy/ss20.html
Alchemical Child's Play, from Splendor Solis
...the allegory of conjunction and multiplication of prima materia
(the step of alchemical transformation that was also called "child's play"): the ingredients "joyfully coupling in imitation of their parents."
The Conjurer, Bosch (look at the little boy next to the table)
According to Dixon, whirligig signifies foolishness (as in the Conjurer above) and it is "used to satirize loose living." I do not know why is that so, and she does not explain. In principle, vertical whirligig was "a device for punishing prisoners comprising a wooden cage that rapidly spins around." This is shown here http://www.francesfarmersrevenge.com/stuff/archive/torture/devices.htm
The cage was spun until the victim vomited. This was very funny. It was still used by the British military in the 19th century, so the fun lasted for four centuries. Dixon suggests that Bosch's "Christ" is, actually, the symbol of folly. Interestingly, both the whirligig and the walking frame can be seen in this ms: http://classes.bnf.fr/ema/grands/131.htm
I found an article on the art history of the propeller: Bosch's "Conjuror": An Attack on Magic and Sacramental Heresy
by Jeffrey Hamburger, Simiolus: Netherlands Quarterly for the History of Art, Vol. 14, No. 1 (1984), pp. 4-23. According to this paper, the propeller represents the Cross rather than foolishness. This does not exlude the allusion to whirligig torture, because the cross was also an instrument of torture. There are earlier images, but none older than 1440.
...Medieval European tapestries show children playing with small whirligigs of a hobbyhorse on one end of a stick and 4-bladed propellers at the other end. http://ezinearticles.com/?Whirligig-Pleasure&id=287816
In "Medieval Children" Orme mentions that in England the whirligig was known since 1060. In Europe, it was called "scopperel" and from the onset was both vertical and horizontal. A nice vertical one is shown in http://www.rijksmuseum.nl/images/aria/sk/z/sk-a-1537.z
(1563)clement found that in Novgorod, the scopperel was used as a toy lance
to joust. I was sceptical, but he was absolutely right. This French ms shows how was it done
The hobby horses were first depicted about the same time the lance whirligigs were depicted, and the two are often shown together; perhaps it was a package deal at medieval toy stores. Don Quixote fighting the windmill might have been an allegory of a child on his hobby horse, with a whirligig (?) - an allusion that is completely lost on us.
I never stop being amazed how things are connected.