For example, let's say that the exchange rate is 100 USD per bitcoin at the moment and that you have tons of people who would absolutely sell their bitcoins if it went below 90. I'm not in the bitcoin game, but I think mtgox offers "stop on loss" features so that people could get out of bitcoins automatically.
If you're sure that you want to be in bitcoins, but you think that there are a lot of scared players who put their "stop on loss" too high, you might be able to provoke some kind of selling avalanche by selling sufficiently many bitcoins at once. The price drops below 90, which triggers a lot of automated selling. This makes the price drop even further, triggering other stop on loss barriers, driving the price to 80 and so on.
With your $5 millions you buy up all those bitcoins at a rate of 50 USD per bitcoin when the avalanche stops. Then people realize what just happened and the price of bitcoins crawls back to 100 because that was more or less the fair market price.
The point is, with $5 million dollars you might have the power to create digital panics, a "flash crash", and use that to your advantage.
I don't know if $5 million is enough for that, and it's not a guaranteed return.