funda media duo scutalia inparia habebat

This is Livy's description of the cestrosphendone or dart sling:

hoc illo bello novum genus teli inventum est. bipalme spiculum hastili semicubitali infixum erat, crassitudine digiti; huic abiegnae breves pinnae tres, velut sagittis solent, circumdabantur; funda media duo scutalia inparia habebat; cum maiori nisu libratum funditor habena rotaret, excussum velut glans emicabat.

Although the description of the dart is clear, the description of the sling isn't. Does anyone know what this contraption would look like or how the dart would be fitted into it?

In this war a new kind of dart has been invented.
A two span arrow has been fixed upon a half-cubit shaft, with the thickness of a finger; three short feathers are (fixed?) to this thing made of fir, just as they are accustomed to arrows, they were given around it; a middle sling had two unequal thongs; with pressure to the greater one, the thong has been freed, the slinger might whirl around the strap
just as a cast out acorn was spurting.

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(Hmm..WORDS says that a span is 6 inches.
http://www.conservapedia.com/Biblical_units_of_measure
says a large span was 1/2 cubit (~9 inches) and a small span was 1/3 cubit (~6 inches). So anyway it sounds like the arrowhead was at least 12 inches and the shaft was 9 inches.
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maiori could be dative ("with pressure to the greater?") or ablative ("with greater pressure?") I'm thinking the former and that libratum, being neuter, has an implied subject of scutale. Also I'm thinking that funditor is subject of rotaret, with habena ablative.
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But back to your question. It sounds like there are two thongs (scutalia) -- I'm guessing the habena is the smaller thong -- I'm guessing the two are actually connected. Pull on the larger one and the dart is ejected by the smaller one.
The dart rotates though. I'll admit my first mental picture doesn't rotate the dart. Maybe the dart is sort of wrapped into the smaller strap, then you pull on the larger strap which unravels the smaller strap, thereby rotating the dart, and then the dart is ejected when you pull the large strap all the way.

Here's a link to a thread about slingers trying to recreate the device; you can scroll through it rapidly and see photos and drawings of attempted reconstructions:

http://slinging.org/forum/YaBB.pl?num=1074713443/0#0

Could the disputed Latin passage in any way refer to the sling's cradle having cords of unequal length, as opposed to the cradle of a conventional sling which has cords of equal length?

On a siege engine such as a trebuchet, the rope on one side of the sling is necessarily longer than the other. Could this be discussing a staff sling?