句子结构分析如下：

By saying postdating a check,（方式状语）I mean you can write the check with a date that is later than the acutal date, （宾语从句）so that it won't become effective until that time. （目的状语从句）

1.常见结构：By saying.... I mean...；  或者I mean...by saying...

2. postdate 可以是动词，意思是：“把......日期填迟；填写......时推迟日期”。postdating a check 是动名词作say 的宾语。

3. with a date 作write 的方式状语；that is later than the acutal date 是date 的定语从句。

4. 本句so that 引导目的状语，不是结果状语。因为从句的动作尚未发生，是将来事件，目的性明确。结果尚未显现，就构成不了结果状语。

By saying postdating a check,I mean you can write the check with a date that is later than the acutal date, so that it won't become effective until that time.

【翻译】我的意思是，你可以在支票上写一个比实际日期晚的日期，这样直到那时支票才生效。

By saying postdating a check，一般不需要翻译出来，因为这个词组跟 mean 的宾语从句内容相同，宾语从句是对这个词组的进一步解释或说明。