I'd guess they are just engaged in the subject enough and confident enough. This could mean any of the following: - the lecture has the right pace for them (t

I'd guess they are just engaged in the subject enough and confident enough. This could mean any of the following: - the lecture has the right pace for them (too smart and they doze off, so I'd say it's smartness+pace) - they are genuinely interested - the teacher is a good match for them