Technically backlighting does not make your photos more flat. But practically this effect will often be the outcome when shooting photos against a strong light source like your flash and like the sun.

You can either accept this as a cool style, or you can make up for it by creating a scene that allows for the introduction of side lighting or by stacking adding a clear foreground and background to your photo.

The awesome examples that I refer to in the podcast are from Good Light! Magazine Issue 09: http://www.goodlightmag.com/issue-09