Butterfly guard is a fantastic place to grab a reverse armbar. If your partner reaches for your head, the key is to be sure their elbow is rotated outward (in the video here, a C-grip is used). Use your neck to keep the wrist isolated by "answering the phone" (pinching your head to your shoulder). As you push outward with your neck, pull inward with your hands toward your body (hence the "reverse" moniker), and use your shin to stretch your partner's arm out and away from you. If you'd like additional isolation, you can pinch your knee on top of their shoulder. Be careful: this one comes on very quickly!