The Marugote A and B
After opening a cylinder with the Gyubera tool, the Marugote is used to create a curve on the side wall. The Marugote can also be used to compress a piece's bottom. Unlike a flat, rubber rib, the Marugote's gently rounded surface will not scrape up clay.

What is the difference between Maugote A and B? A is used to create a bigger curve, and B is used to create a smaller curve (on the side wall of the piece). The smaller round end of Marugote B is the perfect tool to round out the sharp transition from a piece's bottom to its side wall.
Marugore A ($8):
Marugote B ($8):
The Tsurunokubi
Tsurunokubi ("crane's neck" in Japanese) is very useful for shaping the inside side wall of tall or small-necked pieces (such as flower vases and sake bottles) which are too narrow to get a hand inside.
Tsurunokubi ($19):
Opening Tips
Throwing a large-bottomed piece by pulling the clay out with your fingers can be challenging -- but that is not the only way to do it. If large bowls and plates frustrate you, try this method!

First, place your left elbow on your thigh as an anchor. Then, rest your left fingers outside of the clay, and put your right hand over your left hand (resting your left thumb on right hand). Using your right hand's heel (below your little finger, when the hand joins your arm) push out on the clay towards the 11 oclock position (just to the left of center). Do not let the rotation of wheel effect your movement.

The pictures below show a piece with the bottom already opened. However, you can use this method as soon as there is an opening in the center of the clay wide enough for your hand. Once the piece has reached the desired size, it will be easy to compress or flatten its bottom with a Gyubera or Marugote.