trv said:
I would assume the point of such an attack is to buy back the currency at a lower value. So you can have the currency you originally had but you also made a profit through the trade.
Yes. Consider an exogenous, asymmetric shock that causes a trade deficit. The agent (speculator) will try to buy out the bank's foreign reserves. A currency crisis occurs when the government can't bring up their currency (they can't prevent the devaluation/depreciation) by means of increasing interest rates or expanding their reserves. Economies with fixed exchange rates are usually target of speculation.
The speculator makes a bet, or rather, pressures the economy for a devaluation of its currency. In general, devaluation=fixed vs. depreciation=floating rates, but the idea is that the money loses value relative to another (nominal or real). So let's say the economy has a fixed exchange rate (relative to another currency).
Depending on the conditions of that particular economy... the devaluation will either: improve the trade balance (positive effect) or... have a negative effect such as lowering the real income (Laure-Metzler effect), a pass-through effect (in economies in high levels of dollarization), etc.
Since you can't apply monetary policy, then the stabilization occurs by means of capital control.
A lot this effects within different economic conditions are predicted by the Mundell-Fleming model.
EDIT:
What's to say investors won't instead buy a different currency or commodities using the currency under attack.
I'm not sure I understand the question, but I think the idea in general is to provoke a devaluation and to profit from this. The reserves the speculator traded (bought) in exchange for domestic assets will be worth more relative to the domestic currency, since a devaluation/depreciation is necessarily relative to another currency. Basically, the gain is made off this transaction, while the economy is suffering from it.