[zadikian]

It requires the router to support v6. If you want, could do an approach kinda like "happy eyeballs" where you try both.

[tardedmeme]

If the whole path supports ipv6 what is the purpose of the compatibility trick?

[zadikian]

If you want to start using v6 before you're sure that every host supports it. But this isn't strictly needed, could just rely on something in DNS telling you the host supports it (I'm not saying AAAA records cause idk if you'd use them in this scheme).