Is recomposing necessary for trap focus to work?

Question

Trap focusing guide says:

1. Set custom setting #A5 to AF-ON ONLY
2. Set the focus mode to SINGLE SERVO
3. Set Auto focus area mode to SINGLE POINT (9 point, 21 point, 51 point or 51 point 3-D will all work)
4. If your lens has an A/M switch, make sure it is set to A for autofocus mode.
5. Pre-focus the lens for a particular distance. Don't forget that a more convenient way to use this trick is to pre-focus using not the shutter button, but the FOCUS button on the back of the camera can be used. I find this easier and more effective.
6. Press and hold the shutter release ALL the way down, the camera shoots only when the selected focus bracket is in focus.

Now, I can see a clear gap between #5 and #6. If I pre-focus with AF-On and depress the shutter, it should fire readily unless I somehow make it hunt for focus. The instructor then clarifies:

1. Compose your shot and set the focus by aiming the centre focus icon at a definite target at the precise distance you want, for example, a branch where a bird is about to land, or on second base where the baseball player's foot will land. Press the "AE-L/AF-L" button near the viewfinder.
2. This will focus the lens, then let go of AE-L/AF-L button.
3. Turn away, press and hold the shutter button all the way down.
4. Now turn the camera back to the pre-focused point and wait, when the subject gets in the sweet spot the camera will take the shot.

However, another question pops up in my mind. Let's say, I want to photograph a bird which is expected to sit on a branch. Of course, I have to pre-focus on the branch, right? At #3, the instructor asks us to Turn away. I presume, this is tantamount to recomposing the frame. I guess the reason is to forcefully defocus the shot so that the camera does not fire when I depress the shutter button. The crucial point is, at step #4 when I am asked to turn the camera back to the pre-focused point and wait (i.e. the branch where I pre-focused initially), why on earth the camera would even wait? Won't it readily capture the branch itself into focus and fire?

As a side note, even this post, too, couldn't clarify my doubt unfortunately.

Answer 1

WEll, I wrote this originally, and you do not re-focus, after you have pressed the af-on back button to lock focus, you turn away,hold your shutter button down, then turn back to you prefocused spot and your camera will automatically make the exposure. (I learned how to do this on a Yashica af230 film camera and posted it on several forums, and tried it out on my then Nikon D50(This method only works on Nikons}seems someone else has plaguiarised my instructions.

Answer 2

Technically, you would be expected to recompose with the focus spot being above the branch, not on that branch. But yes, any cluttered scene certainly will be problem, like the branch behind the one where the bird is expected.

It sounds like it was more of a thought experiment.

An alternative is to just regularly click the shutter yourself when the bird arrives. You might miss the extended wings landing though...