this looks like a focus on resonance or the mask.
The idea here is that she is feeling the sound vibrate in the cheeks, eye cavities and forehead area. These are the key areas of the 'mask'. You can feel the sound vibrate here. Imagine the human head as a sort of speaker and the body/throat/diaphragm as where the sound is created.
I've had teachers that teach in this way. There are 2 issues with this approach.
1) We don't sing actual pieces like this - so if you ONLY warm up or vocalize like this, you miss out on vowels, throat & jaw position, and consonants.
2) You can actually hinder some resonance when you need more space. Meaning that as you go higher up the scale, you will unnecessarily constrict your voice on pitches where you require a more open throat and jaw position.
Solution: Do exercises that start in this closed mouth position but open to a vowel. Example: mmm-eh, mmm-ee, mmm-oh OR nnn-eh, nnn-ee, nnn-oh
For more info, check out these posts: