Final answer:
The 'Stop' hand signal is crucial for safety when coordinating actions, especially in potential hazard scenarios or precise timing tasks.
Step-by-step explanation:
The most important hand signal to agree on with a helper is the “Stop” signal. This is critical for safety in various scenarios such as when working with high-power electric circuits, in which maintaining clear and immediate communication can prevent accidents.
In a situation where one person stands at a point to produce sound and another with a stopwatch starts and stops it upon seeing and hearing the sound, the “Stop” hand signal immediately communicates the necessity to halt any action to ensure accurate timing or to address potential hazards.