A Ring Cheek piece introduces poll pressure as well creating leverage on the mouthpiece. The 4 Ring has 2 rings on the bottom to adjust the amount of poll pressure and leverage you want.
The Buster Roller link in the middle is shortened to 40mm which increases bar and tongue pressure making the bit sharper than conventional link bits. The increased pressure on the bar will lift the head and the roller will increase pressure on the tongue bringing the chin in. Best suited for a horse with a low head carriage and which leans on the bit.