With most brokers, the term short binary options refers to binary options with a short lifespan, and does not have anything to do with shorting / selling short.
Exactly what is considered a short lifespan will of course vary.
How short are the shortest binary options?
The shortest binary options will mature (expire) within a few seconds. There are quite a few brokers that offer 15 second binary options. If you want even shorter timeframes than 15 seconds, there will be considerably fewer brokers for you to chose among.
Binary options with a lifespan of less than 60 seconds are typically based on forex, but other underlying assets are available too, albeit with a much more limited assortment. You can for instance find super short binary options based on shares, commodities and indices.
If you want to try out super short binary options, the ones based on major currency pairs is a good place to start. The forex market is very active and liquid, which makes it ideal for super fast speculation. The highest liquidity is found for the major currency pairs, such as EUR/USD, USD/JPY, GBP/USD, and USD/CHF.
Generally speaking, super short binary options tend to give very small payouts when they pay.
Successful trading strategies for super short binary options are usually based on trading a very large number of options within a trading day, to bring the daily profits up to substantial levels. Of course, this can also cause substantial losses within a single trading day.
One of the reasons why very short term options pay very little is that it is considered easier to accurately predict what will happen within the next minute than predict more distant points in the future. The shorter the timeframe, the lower the risk of something happening that will have time to snowball into a major change. For many traders, this is a big plus. With super short binary options, there is no point in making an elaborate fundamental analysis of the underlying assets. Instead, the traders focus on learning how to spot trends and predict shifts.
Another plus with super short binary options is that your capital is only tied up for a very short time. The option expires quickly and you either made a profit or lost your money.
Can I use leverage with super short binary options?
The answer to this question depends large on jurisdiction and which broker you are using. When it is legal, some brokers do offer leveraged super short binary options trading. This makes it easier for the trader so make a substantial profit even from the minute pays from each individual contract. When you have more money to put at risk, the combined profits can be larger. Of course, this is also true for the combined losses. Never risk money you can not afford to lose.