Trigger
A standing rule that opens both legs of a route automatically once the spread crosses your threshold.
A trigger is a route, a spread threshold, a position size and an exit condition. When the live spread crosses the threshold, both legs are placed at once; when it converges to your target — or hits your take-profit — the position is closed the same way.
The point is coverage, not speed alone. A human can watch one or two routes; a trigger can sit on all of them, including the window that opens at four in the morning on a coin you have never traded.
Set the threshold with room. A trigger armed just above round-trip fees will fire on spreads that cannot absorb a partial fill on the second leg, and a half-executed arbitrage is a directional position you did not choose to take.