What principle supports maintaining engagement and trust during non-verbal interactions?

The principle that supports maintaining engagement and trust during non-verbal interactions is:

• Keep appropriate eye contact and a relaxed posture

Appropriate eye contact signals attentiveness and respect, while a relaxed posture conveys openness and approachability, both of which are crucial for building trust and engagement in non-verbal communication.

The final answer is $\boxed{\text{Keep appropriate eye contact and a relaxed posture}}$.