I'm trying to understand why the following macro works:

/cast [nochanneling] Arcane Blast
/cast [nochanneling] Arcane Missiles

Everything I read on the wiki and in the official forums says that /cast fallthrough is not possible unless the spell is off the GCD, but if you spam this macro quickly, then Arcane Missiles will cast when it procs...

What are the rules governing fallthrough? Also, is there a more complete documentation of the macro api than wowwiki/wowpedia? Any official documentation from Blizzard?