An improvised-tool application: the pen is not a weapon of injury but a small, hard index point that concentrates your force onto a bone the hand cannot ignore.
Use a pen to create a focused pressure point that opens a controlling grip.
Hold the pen in a hammer grip, thumb wrapped, tip protruding from the bottom of the fist. A hammer grip transmits your body weight into the tip; a pencil grip collapses.
Target bone, never soft tissue. The back of the hand, the top of the wrist, the sternum, the collarbone. Pressure against bone produces an involuntary release.
Drive with a short punching motion, not a long swing. Hips first, shoulder second, arm last. Then transition: the pen buys one beat of freedom; use it to escape.