Extrude Arch like in the video I found a video about Gothic Arch modeling on Youtube using Blender 2.78.

At 1:28 the creator extrudes the vertices of the arch.

However, I can't figure out how it was done. The keydisplay does not show any special shortcuts. The extrude settings on the left seems to be default.

How to reproduce this extruding behaviour? 
Any shortcuts or settings changes? Additional plugins?

I guess some shortcut since there is that small (black) gizmo with the dottet line.
No Addon, plain Blender.

The arch was mirrored.

The mirrored arch is extruded along the normals and meet in the middle. Like the extrusion in the video.

Here the view with only one side editable:


Side note:
Without the mirror if you have the whole arch. You can archive something similar with extrude followed by scale. e then s