This is a coaching cue that can beT used it describe hip/spine position.
What does this phrase actually mean though?
When you are in a strong position, your pelvis is neutral and can hold water (as in a bowl).
When you are in a weak position your pelvis would be tilted anteriorly or posteriorly and it would spill the water.
This can be used in the set-up of a squat, any should to overhead movement etc. Try to keep this visual in mind the next time we call for a neutral spine!