i find that when throwing bowls or vases, you need to pay attention to the structure of the pot - or the foundation of the pot. how the walls stand on the base of the pot.
for vases, i shape from the inside, bottom of the cylinder UP to the rim. for bowls i shape from the top of the cylinder, stroking DOWN to the center with my rib, all on the inside of the cylinder.
with bowls, if you shape with the rib on the inside from the bottom UP & out it's very easy to accidently *kick* the walls off their foundation. it's also SO very tempting to do this! then the bowl sags. i do MANY passed from TOP rim down to the bowl center whe shaping bowls. i never count, but i think it's easily 20 or 30 passes to fully shape a bowl. and i get them pretty thin.
keep the wall of the bowl in mind. what you really want to do is coax the walls over. slightly getting the walls to open up more & more each time. getting there fast breaks this foundation & enables the bowl to sag or get those "woop-d-doo's" at the transition from wall to bottom. i shape all the way down to platters this way if i want. i also start with a wide-ish cylinder, angled outward when i know i want to throw a bowl.
i strive to have no perceptable distinction between wall & bottom of bowl.
i apply slight pressure outward at the bowl rim, slowly gaining pressure to the bottom of the pot. when i'm NOT over the solid pot foundation, i push outward slowly. when i know i'm over the pot foundation (base diameter on the wheel head) i press MUCH harder to smooth that wall-base transition zone.
stroking or shaping the bowl quickly, i feel, puts stresses in the clay that come out as warping after firing. many smaller inducements to shaping the bowl usually leave very small warpage to the bowl.
~ hard to explain. no pictures....
see ya
steve