Refers to something seemingly insignificant or humble that becomes extraordinarily valuable when it is critically needed or used appropriately; invaluable at a crucial moment.
这把看似普通的旧工具，在机器发生故障时，只有它才能解决问题，真是关键时刻一壸千金啊。
This seemingly ordinary old tool, when the machine broke down, was the only thing that could fix the problem; it was truly invaluable at that critical moment.
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