Then I got really freaked out, and that's when it hit me: how much Barry looks like Mr. Potato Head.

1.it 是形式主语，how...从句为真正主语。hit me：使我想起，意识到。

2. that's when...

简单的意思理解：“就在那时；就在此时”，when 从句是描写的主要内容。

语法理解：that's when... = that's the time when...。when 是融合型关系副词，引导名词性关系分句。

体会 that is when...的含义：

That is when I knew he was happy.
我知道那时他很高兴。
Well, that is when I would be getting up to get ready for class.
那个时候我正好起床为上课做准备。
That is when your butterfly comes out of the chrysalis.
那时，你将化蛹为蝶。