Q4. Stuff that lets agile software developers show off what they believe in

YAGNI stands for:

You Arent Gonna Need It

"Always implement things when you actually need them, never when you just foreseethat you need them."

Even if you're totally, totally, totally sure that you'll need a feature later on, don't implement it now. Usually, it'll turn out either a) you don't need it after all, or b) what you actually need is quite different from what you foresaw needing earlier.

This doesn't mean you should avoid building flexibility into your code. It means you shouldn't overengineer something based on what you think you might need later on.

DTSTTCPW stands for:

Do the simplest thing that could possibly work.

It is one of the slogan of XP(Extreme Programming)'s 12 best practices. This is something an XP coach will say when he sees an XP developer doing something that is needlessly complicated. In general, XP says don't overdesign your system.

Link: http://xp.c2.com/YouArentGonnaNeedIt.html
http://www.aka.org.cn/Docs/latest/Extreme%20Programming%20FAQ_Chinese.html