Hi Chenchen
I'm always a little nervous about making suggestions like this one (about slightly opening your mouth) because it's easy to exaggerate the idea with bad results, but in the lesson I could see it was helping a lot. There are both direct, and indirect results from this kind of practice.
The direct result is that it prevents you from clamping your teeth together, which is a cause of tension in your neck.
the indirect result is that because your neck and shoulders are looser, your are better able to draw a smooth bow which improves your sound.
remember, this is a practice method. I'm not suggesting you should perform with your mouth open, rather that you practice with it a little open, so it will show you your tendency to clamp your teeth tightly together. Make sense?