With one hand tied behind one's back的字面意思很容易理解，也就是“把一只手绑在身后”。实际表达的是做某事非常熟练，轻而易举。类似于在中文里，高手常常会对挑战者说“我让你一只手也能赢你”。

【例句】
I can come up with a twenty-page research paper with one hand tied behind my back!
我可以轻而易举地写出一篇20页的调查报告。

She can do it with one hand tied behind her back.
这件事她闭着眼睛都能做!