The Colin Miles 2.5 Ring Petros Williams Gag from Bombers Bits has a 55mm centre piece with rings, which creates tongue pressure if the horse becomes strong and also gives the horse something to play with, encouraging a soft contact and helping to keep the horse's attention on the bit. The Petros mouthpiece rotates, preventing the horse from taking hold of the bit. This can be useful for horses that tend to lean or grab the bit.
The mouthpiece attaches to a post running vertically through the centre of the bit rings (hence the 'Cartwheel'), giving a slight tilting action. This can be useful for horses that are difficult to steer, as the rings press against the horse's cheek when a turning aid is given. It also creates more positive poll pressure, assisting with flexing of the head.
The blue sweet iron mouthpiece encourages salivation, softening and acceptance of the bit because it oxidises, creating a warm, sweet taste in the horse's mouth.
The top ring is attached to the cheekpieces to apply poll pressure, resulting in the horse's head lowering, while the half-ring provides leverage, causing a head raising action. When these actions are combined, it results in a gag action. The 2.5 ring cartwheel is milder than the 3-ring, as it gives slightly less leverage.
Please note that the blue will fade with use.