My brother shorts the US dollar against the euro and the euro against the US dollar in turn, which he does through ETFs (Exchange-Traded Funds), but I suppose you could just use euros to buy dollars, then when the dollar gains in value against the euro, use your dollars to buy more euros until they gain in value against the dollar, then use your euros to buy dollars again... Basically, he's just noticed that when the dollar is up, the euro is down, but they always correct against one another. I haven't investigated this myself, but he seems to be making something on this trade.
Exchange Traded Funds allow you to "buy into" various assets without having to buy in great volume. For example, as I write, gold is over $1,700 an ounce, and you might only have $500 to invest. With an ETF, you could put in as much/little as you wanted, and your fund would do the bulk buying.
Another investment wheeze is the "carry trade", in which you borrow money in some very low-interest currency (traditionally the yen, but these days you can take your pick of a lot more currencies), and invest that somewhere for a higher return, then pay back the loan and pocket the difference minus the measly interest you had to pay. This one would probably be impossible for amateur investors because borrowing money for the very low rate means borrowing in "the money markets", not from Ye Bradford and Binglie (whose rate will be BoE plus their margin).
And how is it determined that a price for a stock is inflated and could do with a correction? Well, whatever the source of the belief, the investor is risking having to "buy back" the share/asset at a higher price, so s/he has to believe in it to a certain extent. Perhaps s/he believes that the company has been manipulating the market and is actually over-valued. Perhaps there is some inside information. Perhaps s/he wants to test how far the bank's national government will go to support the company by buying the shares - that's the cynical view of this short of speculation. But it's really putting one's money where one's mouth is.
Sorry if I am simplifying or leaving out steps; I'm just trying to give a picture of the sort of "market inefficiencies" which investors take advantage of... and how dramatic it can be.
(reelingintheyears, can't believe you think this sort of double-crossing and manipulative behaviour is boring!)