besides the obvious like it only being one more then one syllable and having to actually rhyme and having a complex rhyme scheme to it.
it's about how many syllables they rhyme per bar.
like for example what is more complex some rhyming syllables like this or this
Wrap you in rope and plastic, stab you with broken glass 12 syllables
And have you with open gashes strapped to a soakin mattress 13 syllables
25 syllables he just rhymed in two bars