Ok, description of whats going in the video in short:
First create a rectangle, then draw a line across the geometry (ex. left top to bottom right). Right click on the line and choose "construction" then right click on the line a again and choose "create middle".
Now just left click on the centerpoint and drag the geometry to absolute zero (intersection relation will be added automatically when you release the mousebutton) or attach it to some other geometry using relations.
Of course you can always just drag the rectangle over absolute zero and you will automatically get the "centering" relation. It´s up to you.