Ivanessa, if you play the first five notes in order, C-D-E-F-G, and pay attention to your lips you’ll notice that the aperture they make gets smaller as you ascend. If you’re using a tuner to check if you are centered at the right pitch (attention, the trumpet is to concert pitch, if you don’t know what is concert pitch, google it) you’ll also notice that the position of your jaw and tongue has also to move to get to the right pitch.
I think Greg in these beginning videos, with all the PROCESS DRIVEN approach (later will come the results driven approach), wants the student to just observe what the body is naturally doing to change the pitch without giving you prejudices about how you should do it. He wants you to focus on the feeling and to develop the right feeling and not to conscious manipulate your face muscles and tighten them.