Throwing a pot tall and throwing a pot wide and open are two of the most exciting things to watch. They both require pretty dramatic changes in shape at a crucial moment. The way a potter goes about creating certain shapes makes all the difference between a success and a failure.
Clay has a "memory". This means that the particles that make up the clay respond to shaping according to previous actions. If a potter were to open the body of clay out wide from the get go (think of a wide platter shape), the particles that make up the clay walls would get weaker and weaker because they are being stretched and spread without any "memory" of a tight, compact, strong wall. That is why a potter will keep the walls pushed together as the pot is being thrown (ie. getting the walls nice and thin) until the very last move. That last move is critical. That is the moment where all of your hard work and repetition of pushing those particles together comes into play. In one move, the walls are expanded to the desired shape. Since the particles have the memory of being so compact, they can take the sudden spread and still hold up. If you mess with them or spread them too much after that, they loose the compact memory and spread until they fall apart. And you have a collapsed pot.
In these short videos you can see me throwing a tall cylinder, necessary for any tall vase regardless of eventual shape. In the second, I shape it a little bit. Enjoy!