"Nói Lái" is a form of play-of-words common in the Vietnamese language, both spoken and written, involving transposition of sounds between adjacent or nearby words. It is usually used to inject humour into what one says, and/or to require the listener to think a little before understanding the real meaning.
In most languages, swapping the odd sound around is always good for a laugh. For example, take the spontaneous spoonerisms of Thomson and Thompson. But Vietnamese seems particularly suited for it. Take two words, and just swap around a vowel , a consonant, or even a tone marker for instant innuendo.
Since the examples from "Nói Lái" are (ahem) political, I guess I better settle for one of scatological nature. At first glance, "cây dù" is just a decent clean-living word for "umbrella". But sometimes, it's a way of hinting (via a simple swap the vowels) "cu dầy". That is, "long penis". Of course, you're never going to say those words in in public.
One mixed-language example I have is with my fiancee. A joke we share is saying "Thanh Cù" to each other when we mean "Thank You". But we both know "Thanh Cù" is just a hop, skip and jump from "Cu Thành", which also means "penis". Or rather, "wee-wee".
Nói Lái is very popular here. My fiancee and her brother laughed their guts out when they read the page. As for you, you'll just have to click the link.