Yanfei's charged attack stamina consumption is reduced by 15% per seal. Yanfei's C1 makes each of her seal reduce stamina cost of her charged attacks by and additional 10%. Her C6 allows her to have an extra 1 seal for a max of 4. 4 seals, each giving 25% stamina reduction each results in her charged attacks not costing stamina. Video proof attached, first charge attack with 0 seals consumes stamina and causes bar to appear, second charge attack with full seals after gaining them through Yanfei's e does not.