Ah, another thread of the “don’t knock it till you try it” type. It’s a type of breast stimulation that is highly reactive for both parties, more accurately named 乳首ズリ. Primarily, the nipple is used to massage either the very tip or the back of the tip of the dick which is a very stimulating experience due to those being very sensitive areas. I wouldn’t say it’s widely known but the niche does have a large pocket fanbase within the breast play grouping. I would say, if any would be trying it, that it’s a technique best used during the middle or end of a sexual encounter involving the breasts. If any added stimulation is needed or wanted, a bikini top, sportsbra (regular bras are not ideal do to the underwire), tube top, or cupped hand to cup the dick against the nipple until the desired result. I’d recommend trying it at least a few times if you can.