You can move a control point by dragging it with the mouse, add a new one by clicking the mouse, and delete one by holding down the Shift key while clicking on it.
Each four successive control points defines a cubic curve. That is, controls 0123, 1234, 2345 each define a curve. These curves join together smoothly (the first and second derivatives are continuous).
Unlike a natural cubic spline, a B spline has local control. This means that modifying one control point only affects the part of the curve near that control point. This is very useful when using the B splines for designing shapes.