This is from the Lumisphere website and the L-358 instruction manual located here:
http://www.sekonic.com/images/files/L-358.pdf
1. Incident light measurements can be performed with the lumisphere fully extended or in the retracted position. You can switch between Lumisphere in the up position or retracted position by firmly rotating the Lumisphere retracting ring q until it clicks.
2. When the Lumisphere is raised
This is used to photograph people, buildings, and other three dimensional objects.
Measurements are basically made by the method of measuring with the lumisphere aimed in the camera direction (more precisely, in the direction of the light axis of the lens) at the position of the subject.
3. When the Lumisphere is lowered (flat diffuser function)
This is used to photograph manuscripts, paintings or other flat copy. It can also be used for measuring illumination levels (see page 25), or brightness difference (see page 23).
Lumisphere Retracted Lumisphere