Double Bridles - The Curb Bit is a Nutcracker 

Curb bits are very powerful, second-class levers of the 'nutcracker' or 'lemon squeezer' type. They are literally a 'tongue, lips and bars squeezer'. They are meant to enforce poll flexion with minimal rider effort - through pain or the threat of pain as the mouthpiece compresses the tongue, lips and bars.
Like all bits, curbs ARE A WELFARE RISK
When riding with bits that add mechanical advantage riders may feel the horses are very soft and light when ridden with a curb but they forget the mechanical advantage a lever provides - and don't acknowledge that the extra attention the horse is giving is because of pain or the threat of pain.
You must account for the physics - all levers have a mechanical advantage above 1! (That's why they are so popular). You cannot compare the rein tension applied to a snaffle which is not a lever, with the rein tension applied to a curb UNLESS you also account for the mechanical advantage. That is, you calculate the mechanical advantage according to your curb's dimensions (check out the video), and multiply the rein tension measure accordingly - It's not rocket science. It's middle school physics).
The moral of the story?
Horses are at risk - because riders can easily underestimate and be oblivious to the amount of pressure the horse is receiving on sensitive tissues.
Plausible deniability is not an excuse for causing harm with bits (i.e. claim it is too hard to know what you are doing). Riders should know they have a powerful lever in their hands and therefore:
• Riders MUST NOT push the horse into the curb rein contact.
• Riders MUST NOT keep tension on the curb rein when the horse is already flexing sufficiently at the poll.
• Riders MUST NOT use the curb to bend the horse laterally (that's what the snaffle/bridoon is for) nor to give slowing-shortening aids (that's what the snaffle/bridoon and the rider's seat/core are for).
• Riders MUST NOT continue pushing horses into strong contacts (regardless of the bit), in order to obtain more and more extravagant leg movements - there's a very sensitive mouth to respect.
Instead...
• Riders MUST allow the curb chain to disengage as soon as the horse has flexed at the poll - before the horse comes behind the vertical.
• Riders MUST use the curb as if riding one-handed (when you pull on one curb rein both shanks go backwards together).
RIDERS SHOULD AIM FOR THE SELF-CARRIAGE - TRAIN ETHICALLY TOWARDS BEING ABLE TO PERFORM WITHOUT A BRIDLE.
Curb bits have the mechanical advantage of a powerful second-class lever of the NUTCRACKER type.
Remember Batman's words: "With great power comes great responsibility"