I've got a poi, a ball on a string, and I've got a bucket of water. And you're wondering what a poi and a bucket of water have in common. I'm going to use them for my next demonstration. If I take the poi and I swing the ball around in a horizontal circle, then I'm applying equal amounts of tension to the string all the time to get it to go around in a circle. I'm applying a centripetal force. On the bottom, I am applying a centripetal force on the ball to make it turn. If I make the ball go in a vertical circle, then I have to apply a lot of force at the bottom to make it turn, but at the top, hardly any at all. In fact, non-gravity itself then allows the ball to go over. At the bottom, I have to make the ball turn, and I have to act against gravity. At the top, gravity is quite happy to supply the centripetal force that I need. Now, a bucket of water, if I spin it in a vertical circle, it's quite happy the water will stay in the bottom of the bucket because gravity's pulling it down. At the top, gravity's going to pull the water down as well, but I don't want it on top of me. I want it to stay in the bucket. So, I have to get enough velocity up so that the centripetal force of the bucket pushing against the water is greater than the gravity force so that the water will stay in the bucket. If I do that, I can spin it reasonably happy and the water stays in the bucket. However, if I don't spin it quite as fast as I should, the water comes out of the bucket. So, it's all the matter of applying the right velocity in the right place at the top to make sure that the water stays in. At the top, the bucket is pulling around, pushing against the water, more than gravity is pulling it down. So, there is a reaction force between the water and the bucket.