Why the octave closes the door
The first eight lines rhyme ABBAABBA. That pattern uses only two rhyme sounds, and the second one repeats immediately after the first, so by line 8 the ear has heard every sound the octave owns. Nothing is left open. This is deliberate: a Petrarchan octave is not a question waiting for an answer, it is a room with the door shut.
The alternative rhyme scheme, ABABABAB, would leave the quatrains alternating and the ear still expecting something. Sestets that answer a closed octave work; sestets that continue an open one drift. The eight-line unit is the argument, and it should be finished.
The volta is a change of footing, not of subject
Line 9 is where the turn happens, and the most common mistake is to treat it as a change of topic. Nothing breaks in a good volta. What changes is who is speaking to whom, or what the same facts are now taken to mean.
In the specimen above the octave complains that the garden defies the gardener. Line 9 does not abandon the garden; it stops asking the garden to obey and starts reading the disobedience as a method. The subject is identical. The stance is opposite. That is a volta.
The sestet has three rhymes to spend
CDECDE gives you three rhyme pairs across six lines, and the shape is not symmetric on purpose: the first of each pair is separated from its partner by a line with a different sound. That delay is what keeps six lines from feeling like three couplets.
Counting syllables in the specimen: every line falls in the range of ten to eleven syllables, with the stresses landing on the even positions in the phrases that carry the argument. Two lines are loose — line 13 opens on a stressed syllable — and the looseness is audible as a small push at exactly the point where the poem is closing. Strictness everywhere makes the ending flat.
Working rules
Write the octave first and do not allow yourself a sestet until the octave can be read as a complete statement. Then write down what changed between lines 8 and 9 in one sentence, in prose. If you cannot write that sentence, there is no volta yet, only a longer poem.