This is taken from the University of Nevada:
ms Relative to Bit Design:
Curb Bit
: Any bit regardless of
mouthpiece style that has shanks
. This is a leverage
bit.
z
Curb strap/chain
: An accessory to a curb
bit. Provides for
normal function of the
curb bit by restricting degree
of movement of
the mouthpiece. Applies pressure at
the chin groove when reins
are drawn tight.
Leverage Bit
: Jointed or solid mouthpieces
attached to
shanks
‘Shanked snaffle’ is an oxymoron, as you are calling that bit both a leverage and anon leverage bit, based on mouth piece design, not function
It is a convenient slang, as it is much easier to describe a curb with a jointed mouth pieces that way, thus the term is used both by tack catalogs and professionals, but the professional sure know that in reality they are talking about a curb bit
The true Tom Thumb, that has fixed shaNks that are aLSO STRAIGHT UP AND DOWN shanks, is a poor choice in a curb bit,but not because the mouth piece is jointed, but because both those relatively short shanks, that are fixed, and straight up and down design give very little if any signal between the time the curb strap is engaged and bit contact
Signal time in a curb bit depends on ratio between purchase and entire shank length, angle of shanks and whether shanks are loose jawed or fixed
Short shanks have less signal then longer shanks, but also a decrease in severity, far as leverage, hands being equal, thus you balance shank length on how educated that horse is to a curb bit,
In other words, a horse that is at the level of responding to very light rein, gets increased signal from a longer shanked bit, while a horse not at this level yet, benefits between a little less signal , but less curb action (leverage )
Moving a horse up from the snaffle , AFTER he has all the basics on him, to a curb with a jointed mouth piece, loose jawed, short shanked, just introduced that horse to slight curb action, using a mouth piece that he is familiar with
Moving to a curb with a port, fixed or un fixed shanks or first going, or even staying in a curb with aj ointed mouth piece, depends on the individual horse;s preference and ultimate end goal, far as discipline
Again, for ease of differentiation, often all curbs with aj ointed mouth are called Tom Thumbs, if not by my other least favorite terminology, 'shanked snaffle!