How do I extrude so that the circle moves inwards like at 13:04 in <https://www.youtube.com/watch?v=xU_pi1UxISs>? When I do it the vertices just move up. 

![before](https://i.stack.imgur.com/ErJFO.png)

![after](https://i.stack.imgur.com/Fmiwu.png)