A chambon is similar to a martingale in that is designed to prevent a pony from raising his head beyond a certain point. It attaches to the pony's girth or waist strap on one end and then, like the running martingale form a "Y". However, unlike the running martingale, the two arms of the "Y" run through an attachment to the browband of the pony's bridle and along the cheeckpieces before attaching to either end of the pony's bit. Alternatively the arms of the "Y" may go through the bit rings and then attach to each other.
The diagram below illustrates the positioning of a chambon on a bio-equine:
The chambon is designed for use when longeing a bio-equine (as opposed to riding), so it is quite well suited for ponyplay. In my opinion, the chambon is even better than a martingale for controlling head carriage in spirited ponies.
If you enjoy riding your pony however, you will probably prefer to use the martingale during ponyplay.