Cho Hakkai's Cuffs
Cuffs 【カフス】 (Sai. Vol 1 - Continuous)

☆ The three silver fasteners around Hakkai’s left ear. They’re youkai power limiters, and if they’re removed Hakkai changes to his essential youkai form and his youkai power is unleashed. Unlike Goku, even if he takes off his limiters Hakkai retains reason (well, Goku is different). Hakkai still hasn’t been affected by the Minus Wave; he wears the limiters simply to repress his huge youkai powers. Hakkai has strong enough youkai powers that it finally took three power limiters to suppress it.