When a Mech gets close enough to Trip to pick her up by her head, she will use an EMP to disable the mechs attacking her. You then must destroy the mechs. Most of these scenes are automatic, and don't happen often at all. Listed are the few times this happens:
- Early on in Chapter 2 (automatic)
- When you take over the turret (let the mechs get close to her so she uses EMP
- Late in Chapter 2 (automatic)
- Chapter 3 while on the turret (let the mechs get close to her so she uses EMP
- Near the end of Chapter 10
If you still don't have this by the end of the game, I suggest replaying Chapter 2 again.