Select a point or an edge to "drag" using the cursor. How the model will change depends on both what you select, and existing constraints and dimensions on the model. You can drag a group of edges by preselecting them before choosing this tool. For finer editing of the shape of the spline, you may want to use the "spline Edit" function instead.
Note: When using this function with a large number of items it is recommended that you first turn off Auto Constraints : Global. This will both significantly speed up the function as well as avoid the creation of unwanted constraints that could alter the results.