HFT arbitrage

With extremely fast orders what HFT tries to achieve is take advantage of the bid-ask spread for essentially a risk-free trade. For example, in the bitcoin market, if bid orders are being hit at $6,400 (by sellers) in quick succession, the probability is that price will keep going down to $6,350 (providing there is more ask depth than bid depth). What a HFT programme will do is fire a sell order within microseconds of that $6,400 bid order being hit, just as it's being withdrawn from the market. This puts them in an immediate breakeven position and essentially a risk-free trade.