While handling my GPO this afternoon I noticed that there is a small hole at the center of each suction disk. When I touch the disks, what appears to be a valve like disk below the hole pulls inwards, allowing the hole to open up. I took a rough picture of it but I can't post it right now. I would've taken a video for better clarity but it was hard enough to get the picture. I think I have an idea as to the purpose of this hole, but could anyone tell me more about it and its function?