The mere expectation or rumor of a central bank foreign exchange intervention might be enough to stabilize the currency. However, aggressive intervention might be used several times each year in countries with a dirty float currency regime. Central banks do not always achieve their objectives. The combined resources of the market can easily overwhelm any central bank. Several scenarios of this nature were seen in the 1992–93 European Exchange Rate Mechanism collapse, and in more recent times in Asia.
In a currency pair with a wider spread, such as the EURCZK, the currency will need to make a larger movement in order for the trade to become profitable. At the time of writing, the bid price for this pair is 25.4373, while the ask price is 25.4124, so the spread is 0.0200, or 20 pips. It's also not uncommon for this currency pair to have movements of less than 20 pips a day, meaning traders will likely need to perform a multi-day trade to make a profit.