The Difrax straw cup has a leak-proof system in the straw. When your child slightly pinches the straw with their lips while sucking, the leak-proof system in the straw will open.
It may take some time for some children to get used to this, as it is different from a 'normal' straw. It can help to try a few times.
For new cups, it may also help to pinch the straw at the valve a few times to ensure that the silicone no longer 'sticks' due to its newness, making it easier to open while drinking.