Answer
Quickly adding water without circulation may case it to cool unevenly, causing uneven thermal contraction in different parts and potentially causing pieces to break.
Work Step by Step
If water is added quickly, part of the hot engine is cooled more than nearby parts. There will be more thermal contraction of the first piece compared to the second. This mismatch causes internal stress forces between the sections, possibly cracking the radiator or even the engine block.
This effect can be mitigated if the engine is running, and circulating water, which assists the various parts of the engine to cool uniformly.
