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you can sell off some (not all) of your BTC,
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I believe this is called shorting.
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I believe this is called "selling".
Unless I'm mistaken, "shorting" would involve selling bitcoins that you do not own.
You borrow bitcoin from someone else, and then sell those borrowed bitcoin with the intention of buying them back later at a lower price and returning the re-purchased bitcoins to the lender.
Example:
Borrow 100 BTC
Sell the borrowed bitcoin at $66 per BTC for a total received of $6,600
Wait for exchange rate to drop to $50 per BTC.
Buy back 100 BTC for $5,000 (keeping $1,600 for yourself)
Return the 100 BTC
The issues you can run into with shorting are that the lender may want the BTC back before the price has dropped, or the lender may require some amount of interest rate on the loan and the interest can end up being higher than the revenue from shorting.
A relevant quote from John Maynard Keynes after being wiped out in a short position in 1920: "The market can stay irrational longer than you can stay solvent"