Open Question: Why can China keep its currency, the yuan, artificially low?

is there an agreemnet between the countries? other currencies are at the marcy of the market. japanese yen has been buying , rose by 50%, which definitly hampered their economic growth, its economy is still at the bottom, china's econmy has been robust, because of its cheap labour, government's strong initiative, artificialy low currency. why only China has been pampered ?