When speculators anticipate an appreciation, they buy the currency when it is relatively cheap and sell it when it should rise.
For example: in February, the dollar is equal to 110 yen. The speculator thinks that the dollar will appreciate in April, so he spends 1.1 million yen to buy 10,000 dollars. In April, as he expected, the dollar is 120 yen.
When speculators anticipate that a currency will depreciate, they sell the currency in the foreign exchange market when the price of the currency is relatively high. When the currency actually falls, they buy the currency and earn the difference.
For example, in the above example, the speculator believes that the US dollar will depreciate in April, so the investor will sell the US $10,000 in his hands and get 1.1 million yen. In April, as he expected, one US dollar is equal to 100 yen, and the investor will sell the Japanese yen in his hands and buy US $11,000, so he earns 11,000-10,000 = US $1000.