So let’s take our hypothetical 1 BTC from before and take a short position on Ethereum. We are able to borrow 2.5 BTC worth of ETH and sell it. 30 minutes later, the price of ETH has plummeted 10%. Now we can close our short position, buying back 2.5 BTC worth of ETH; except now, since the price has dropped, we are buying more ETH than what we sold. Our borrowed coin can be payed back and we take the rest as profit!
Cryptocurrency is volatile! There is always the chance that the market will crash, or that you will face some other catastrophe. Cryptocurrency isn’t a centrally controlled and regulated fiat currency. If you lose a coin, or someone cheats you, or your account gets hacked, there is nothing you can do about it (which is one reason why you want to have 2-factor authentication set-up).
If you have an account on Poloniex.com or Bittrex.com (and other crypto exchange sites) you can use their API with TabTrader to easily trade and monitor prices on your phone. It's important to me that the app connects to Poloniex and Bittrex because these exchanges have good volume. And they're pretty credible. TabTrader supports other major exchanges too :)
Only invest what you can lose. During the recent crash in January 2018, hobby-investors got burned. Reports of frustration and losses came at the cost of broken monitors, smashed laptops, and heavy monetary losses. While the rules are in more particular order of importance, it’s safe to assume that this is the most important rule, the rule to rule the rules. As soon as your money is converted into cryptocurrency, consider it lost forever. There is absolutely no guarantee you can get it back. Losses don’t simply come from dips in the market; extraordinary factors such as hacks, bugs, and government regulation can mean you’ll never see any of your money again. If you are investing money you can’t afford to lose, you need to take a step back and re-evaluate your current financial situation, because what you’re about to do is an act of desperation. This includes: using credit cards, taking out mortgages, applying for loans, or selling everything and traveling the world (as glamorous as that sounds).