Final answer:
One should never leave a jack under a load without using safety supports or jack stands, as it is essential to secure the load to prevent accidents.
Step-by-step explanation:
The phrase 'Never leave a jack under a load without' is typically completed with the term 'safety supports' or jack stands. When elevating a vehicle or any heavy object, it is imperative to use additional support devices to secure the load after lifting it with a jack. Relying solely on a hydraulic jack poses a risk because it can fail or lose pressure over time, leading to a potentially dangerous situation.
Safety supports or jack stands are designed to hold the weight securely and should always be utilized when a load is lifted off the ground and someone is working underneath it.