It's more like aggressive and risky investing really.
Buy up all the copies of (x) card below a certain price, then once the decreased supply - assuming copies don't flood onto market in response - increases the price you can sell the ones you bought and make a fairly significant profit. Much easier to do with rares, especially newer ones (Like Dregn when he came out), since people will generally have significantly less of them.
Of course, if you underestimate the supply sitting in peoples' collections or lack the clintz to buy enough you'll just be wasting money and probably devaluing the card by encouraging everyone to sell now and buy it back later once the price normalizes.
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