Originally Posted by Skippy!
Technically, they are Shank bits with Snaffle Mouths =)
Technically they are NOT snaffle mouths - as snaffle simply means any bit with DIRECT pressure as opposed to curb which means any bit with indirect pressure/leverage.
A tom thumb has no snaffle rein and therefore regardless of the mouthpiece is NOT a snaffle bit or a snaffle mouth. If there were reins attached at the same point as the mouthpiece - that would be considered a snaffle rein, and this would then be considered a COMBINATION bit - however this is neither a snaffle mouth nor a combination bit as it does not work in multiple ways (direct + indirect pressure) on the horse.
The multiple effects on the horse's mouth come from the BROKEN or SINGLE JOINTED mouthpiece (often mistakenly referred to as a snaffle b/c of it's common use on a direct-rein pressure bit) which causes nutcracker action. Because the broken/single-jointed mouthpiece is on a shanked/curb/leverage bit, the resulting effect on the horse is both nutcracker action on the jaw (again not snaffle action as there is no direct rein causing the pressure) coupled with leverage action on the jaw from the curb chain and slight poll pressure from the leverage of the shanks being drawn back from the mouth.
Hope this helps clarify - sorry to be such a stickler but it kills me when people refer to anything with a jointed mouthpiece as a snaffle as that is not correct. Similarly, even a mullen mouth (mouthpiece with no joints) that has reins that attach at the same level of the mouthpiece and works on direct pressure is considered a snaffle, despite the lack of any joints in the mouthpiece.