when the computer takes over, take your fingers off of the buttons. don't touch anything until it says to hit Z. then hit Z and release, it will ask you to hit Z a few more times, and then it will tell you when to flip the wii mote up.
I failed about 6 times before someone on here explained it in depth to me. just make sure you're not touching any other buttons, and you're just touching Z when it asks for it, not holding it down.
You're both referring to separate parts. The "computer takes over" and you have to push Z on your way to the 1st ranked assassin. The person who made this topic is referring to the side mission bike jump job, where the computer does not take over, just make sure to flick the remote with enough time for it to actually register (bike takes time to jump).