This is a concept known as scalping, or spread limit trading. It works well on small exchanges such as Idex, in sideways markets. You can profit on an alt coin without it moving a single satoshi in price. (This is for educational purposes only, and should not be considered financial advice, always do your own research)
So the theory goes like this...
You are looking for mid cap alt coins on coinmarketcap, typically between the 200-500 rank by market cap. You are looking for coins on the exchange that have a high spread (the difference between the highest bid and the lowest ask). In my trading I look for coins with a 3-5% spread, with a min of a 2 BTC daily volume.
When you find one of these coins, you want to look at their chart, and their orderbook. You want a chart that is relatively stable, sideways, but volume is not low. You want an orderbook that has 1) More buyers than sellers 2) No Whales with buy orders or sell orders close to the current price (always respect the bigger fish or he will eat you) 3) No signs of anyone spoofing (placing a large buy or sell order in attempts of changing market sentiment with the intention of canceling it before its filled) and no signs of market manipulation.
Lets say the highest meaningful bid on the coin you find is 1,300 Satoshis (a non-meaningful bid is a microcosmic amount that has no impact on the overall standing of the orderbook). You then place a limit buy order at 1,301 Satoshis, and wait for the order to be filled, adjusting your limit order as needed for fluctuations in the market. With luck, your order will be filled.
Next, take a look at the lowest current and meaninful ask, lets say it is 1350 satoshis. You will place your limit order right around this market, adjusting as needed for fluctuations in the market. Now, you play the waiting game...
This concept is profitable by being more patient than the other traders in the market. All it takes is one inpatient trader to come along (Trust me, there is plenty of them!), and buy at the market price, which will use the price and order of the lowest current ask (your sell order if you did things correctly) and will start filling their buy order from there. If done correctly, your order will be filled and your coins, sold.
You have just profited 3.8% by being more patient than the trader who placed the buy order, and have profited without the coin moving a single satoshi in price (in theory).
You can set stop limits if you see the coin becoming increasingly volatile during your trade. Also as a general rule of thumb, never risk more than 5% of your trading bankroll, ie. If you are using a 1 BTC bankroll, you should be making .05 btc trades.
With practice, you can work up to running multiple of these trades at any given time.
A friend of mine who trades full time on Idex, consistently profits .1 BTC a week using a .5 BTC bankroll and scalping.
Once again, this is just theory trading, just because myself, or another trader is successful with this method, does not mean it is the method for you. If you are interested, try trading on paper with it, ie. write down all what you would have done without actually doing it, and seeing how the outcomes turn out, or try with a 100k trade.
I hope this information is helpful (its not like I accidentally hit refresh and had to type this all over again or anything... pssshhhh, never happened)