hand shaping a U, moving slowly towards the sides of your chin

We do it slowly because it's a long vowel and we level our hands with our mouth to better connect lips and hands movements.

Make a U with your hand and slowly move it towards your chin till you touch the sides of it with both ends of your hand-made U. The U shape is to make us remember that this sound is very rounded and quite closed (a bit similar to the sound of the letter U in many languages). The movement of the motion from the front backwards means that this vowel is pronounced deep inside our mouth, to the back.