There is a strap attached near the top of the centre rear seat. Pull this strap to fold out the rear centre armrest.
The centre armrest can provide additional arm support but also contains a hidden cup holder with two slots.
To access the cup holder, push the top section of the centre armrest and let go. The sides of the cup holder open up. When the cup holder is no longer in use, gently close both sides by pressing the indentations in the centre of the edges until you hear a click.