At 10:00 in, the teacher is able to snap the uneven vertice that's below the grid back to the exact level as the rest of the vertices. How is that done with a shortcut?

I'm using the 3D cursor as a pivot for scaling all vertices to 0 in the Z axis. At it's default position of 0,0,0 in world space, the result essentially flattens the vertices vertically on the grid floor.