Wierd Face after loop cut

Hi Can someone explain why I don’t get / shape and get this shape like the SQUARE ROOT SYMBOL?

  1. I loop cut a cube (vertical center)
  2. I Pushed the loopcut on the Y Axis so the whole shape became like >>.
  3. I pushed down the upper vertice of the head on Z axis.

This means I should get a VALLEY shape.

I am not entirely sure what you want to do.
I recreated it and got to the chevron shape:

But then you move the vertex downwards and end up with non-planar quadrilaterals:

I converted those faces into triangles, but is this what you want?
image

You really cannot expect to get the correct shape when faces become non-planar. That is, not all vertices of a certain face are in the same plane, some have been moved out-of-line. Converting to triangles ensure that faces don’t do this, as triangles are unable to have the same issue.

I don’t understand what you mean by valley shape, exactly. Can you provide an example of what you want?