What techniques are being used here to influence the subject to pick a certain outcome? The question is about this video. 

Summary of the Video: (can skip if you watched it)

The only audio is "elevator music." There is a man who communicates only with written orange signs. He asks the viewers to quickly perform some easy arithmetic calculations, and he shows the answer to each one after a few seconds:

$5+3 \ \ = 8$

$9+2 \  \ =11$

$10-6=4$

Then, he asks the viewer to quickly pick a number between $5$ and $12$. At this point, I suddenly felt a sense of urgency, and rapidly picked $7$; it was the only number I could think of in that second. Of course at the end of the video, the man guessed that the subject picked $7$. 

Question:

How does this video influence the viewer to pick $7$? More generally, what techniques do these types of videos employ to try to manipulate the subject into chosing a certain outcome?



I tried to look for certain patterns in the calculations and I found a diagonal symmetry, but I'm not sure if it's relevant: 

$\color {red}{\text{odd}}  \ \ + \color {red}{\text{odd}}$

$\color {red}{\text{odd}}  \  \ +  \color {blue}{\text{even}}$

$\color {blue}{\text{even}} \  +    \color {blue}{\text{even}}$

I've also noticed that the average of the $3$ results is $\dfrac {8+11+4}{3} = 7. \overline {6}$
Priming is a phenomenon where an exposure to a stimulus affect/influences response to another stimulus. It is however, a broad definition which may or may not fully explains what actually happened in the video.

For what's it worth, I notice that number 7 was the only number (between 0-9) that didn't appear at all in any of the arithmetic calculations, answers or questions. But I can't say that I understand how this could influence subject's choice.