Final answer:
When drawing with the pen tool in graphic design programs, pressing the Alt/Option key lets you change individual control handles for precise manipulation.
Step-by-step explanation:
To change the control handles while drawing with the pen tool in graphic design software such as Adobe Illustrator or Photoshop, you should press the Alt/Option key. This allows you to manipulate a single handle independently without affecting the other, giving precise control over the shape of the path's curve. Using this key in conjunction with the pen tool can significantly enhance the flexibility and accuracy of your vector drawings.