I was watching a video on retopology and saw the uploader extrude a mesh face in a way that I've not seen before. He's in edit-vertex mode and has a partly completed mesh with an L-shaped bend along its edge. He then selects the vertex on the inside corner of the L and then appears to click somewhere further out - and this causes an entire face to be created:
YouTube clip: https://youtu.be/KNjCaizeal0?t=8m29s
I've been reading through the docs, but can't figure out what key I need to press to do this. Does anyone know how to do this?