Like any other loan, this borrowed Bitcoin must be paid back with interest. On losses, you will need to pay back the loss and the interest. Poloniex offers up a great guide to margin trading that explains everything you need to know. It’s worth reiterating that the estimated liquidation price is the price at which a forced exit from our position would occur, costing us all of the Bitcoin in our margin account so that it may be used to pay back the borrowed coin. Utilizing stop limits to avoid this is almost always a good idea.
Always pay attention to Bitcoin. Most altcoins (every cryptocurrency except Bitcoin) are pegged more closely to Bitcoin than Asian currencies were to the USD during the Asian Financial Crisis. If Bitcoin price pump drastically, altcoins price can go down as people try to exit altcoins to ride the BTC profits; inversely, if Bitcoin prices dump drastically, altcoin prices can go down, too, as people exit altcoins to exchange back into fiat. The best times for altcoin growth appear when Bitcoin shows organic growth or decline, or remains stagnant in price.