EAs run on the trader's terminal or a hosted server and execute continuously. They remove hesitation and enforce rules, but they also execute a flawed idea faster than a human could, and many commercially sold EAs rely on martingale or grid logic that produces a smooth equity curve until a single trend destroys it.
Backtest quality is the usual weak point: modelling ticks from minute bars, ignoring spread-widening and using ideal fills can turn a losing system into a profitable-looking one.
Example: a grid EA doubles size after each loss. After eight consecutive losers, the position is 256 times the original. A $10,000 account starting at 0.01 lots is holding 2.56 lots against it and is stopped out.
Related: metatrader, backtesting, latency-arbitrage, risk-of-ruin