See this answer for a basic explanation.
Since it's a little vague (which I never noticed, seeing as I thought it made sense. Thanks for the feedback, Trach :P), I'll explain it in words.
Look at the article, there's a link (between the two questions, that is.):
"Damage is calculated when the attack occurs."
According to the code of a battle scene, every function for a move, not just damage, is calculated when the move is directly initiated.
In summary: Sketch will not copy a move until the ACTUAL ATTACK is used.