Ironically, you can sometimes get more control by restricting the motion. You'll try limiting the motion of the control handles to a particular axis, and you will set the control point so that only one control handle moves at a time:
In Graph Editor, zoom into the keyframe on frame 19 where you stretched the cube, select one of the handles with the RMB, press the G key to grab it, and move it around with the mouse. Tap the Y key. Move the cursor around and observe how this affects the movement.
Press the X key. Move the cursor around and observe how this affects the movement. Press the RMB to release the curve handle without making a change.
Now, select the control point in the center of the control handle.
Press the V key. Select Free from the drop-down menu, as shown in the following screenshot. Pick one of the control handles and move the cursor around and observe how this affects the curve.
