Final answer:
Opening an acetylene cylinder's valve more than necessary increases the risk of a rapid gas release, making it difficult to quickly shut off in an emergency. Leaving the wrench on the valve poses a hazard as it may inadvertently open the valve further. This is akin to increasing leverage with a wrench which makes tasks easier but also introduces risks.
Step-by-step explanation:
The question pertains to the potential hazards of improperly using an acetylene cylinder's valve during welding or cutting operations. Opening the acetylene cylinder's square stem valve more than the recommended number of turns can set up a dangerous situation. Specifically, only opening the valve a quarter to a turn and a half is generally suggested for such operations.
Overopening the valve to say three-and-a-half turns could lead to a rapid release of gas which, if there is an accident or the need to quickly shut off the flow, may become cumbersome, risking delay and potentially allowing a hazardous amount of gas to escape. Furthermore, leaving the wrench on the valve can pose a hazard because it could accidentally be bumped or knocked, further opening the valve and releasing a dangerously high volume of gas.