Tip: When you know you will be collaring in to close the form, it helps to throw the cylinder in a cone shape rather than straight up and down. The inward slope of the original wall will give the clay less stress, since it doe not have to move inward as far.
Once the form has been completely closed, it can be pushed around in ways that are not possible otherwise. This is due to the air pocket you have created inside the form. The pressure of the air will keep the clay from collapsing. You have, in essence, made a clay balloon.
Push the clay into the shape you desire for your completed jar. (Do be careful not to push the limits too far, since clay balloons can also pop.) It is good to keep in mind where you will be cutting through the closed form later to create the lid. As you can see, I chose to not only to create a sharp shoulder transition but also added texture to the main body. I repeated the slip-created texture on the asymmetrical coil handle I attached while the form was still on the wheel, as well.