I dreamt that when you die, you end up flying in darkness. But in the midst of it

[Belacqua]

Some poets play with the structure. I have a book of haikus and many of them deviate from 5/7/5.

True, but isn't haiku always 17 or fewer syllables?

Traditional Japanese haiku really have to have 17 exactly. The form is very strict. There has to be a "cutting word" at the end of one of the lines, and a seasonal word that tells exactly what time of year it is.

https://en.wikipedia.org/wiki/List_of_kigo

The type of subject is also very limited. It has to be a snapshot with no explicit lesson, moral, or epiphany. It has to be something close to nature.

Poems which are similar but don't strictly follow these rules are called senryu. These can be more like jokes or pointing out amusing things. 

That said, there are various schools which stick to tradition with more or less seriousness. All these Japanese things -- flower arrangement, tea ceremony, sadistic rope bondage -- have schools and traditions and when you study you have to choose a master. Experts can identify from the way you place the stems, rotate the tea bowl, or tie the knots, what master you're following. 

Modern haiku schools are looser in what subjects they allow, but I've never seen one that claimed to be a haiku but didn't follow the syllable rules. Santoka, for example, shocked the old folks by mentioning jazz music in one of his haiku. But he kept the other rules. 

The way they're written doesn't emphasize the three lines, as English translations do. But there are three distinct phrases in each.

「渋いとこ母が喰いけり山の柿」= literally: "astringent part mom eats mountain persimmon." Shibui toko haha ga kuikeri yama no kaki. Tidied up for English would be: "Mom is eating the astringent parts; mountain persimmon." Mountain persimmon is the season word, because we know when they're ready to eat. The unspoken part is that mom is giving the sweet parts to her child.