Metal rings have a long history of being used in Yau Kung Mun, Hung Gar, and other styles for weight training, to harden the muscle, skin, or bone, or strengthen the arms and fists.
To get the most out of your forms training, martial arts practitioner and writer Stefan Verstappen suggests that forms training be done with “brass or steel rings ...around the wrists and ankles to add weight during the performance of the form.” Verstappen states that “[s]pecial wrist and ankle weights can help in the development of muscles, and endurance.
[1] Ying Ching Kuen and external forms of Yau Kung Mun are practised with iron rings to build strength, power, and endurance.
"[3] Hung Gar is derived from the Shaolin Temple kung fu system developed during the Ching dynasty.
To do fist training, students wear iron rings, weighing from 2-4 lbs on their arms.