The interactive Schlegel diagram viewer has an additional "Pick Vertex" mode,
allowing you to manipulate the Schlegel projection.

Since the central point of the projection (let's call it V) lies outside the
projection plane and is therefore invisible, its position is controlled
indirectly.

Imagine a point P lying in the interior of the projection facet (it always coincides
with the origin of the visual coordinate system) and a ray PV pointing to the outside
of the polytope. Now you can influence the position of V by

- changing the zoom factor: The direction PV is fixed, V slides along this ray.
  Zoom factor 0 makes V coinciding with P (therefore forbidden),
  while zoom factor 1 corresponds to the maximal allowed distance PV by which
  the Schlegel diagram remains a valid polytopal complex.

- dragging a vertex of the projection facet (incident to black edges):
  The vector PV is translated in the opposite direction.

- dragging a vertex of other facets (incident to green edges): The point P and
  the distance PV stay fixed, the ray PV is inclined to "catch" the new
  projection of the point being dragged.

The message "can't drag beyond the projection facet boundary" appears when the
movement can't be continued without violating the polytopal complex property,
that is, a projection of some point from the polytope back side has
reached the boundary of the projection facet.

Note that the "Pick Vertex" mode is active from the very beginning, unlike the
standard javaview behavior, where it starts with "Rotate, Orbit". Please refer to
the general javaview help how to switch between the major modes.
     
The New Projection Facet button allows you to display different projections: 
  Mark some vertices (using 'm' and 'u' keys), then click on this button.
  A new window will be opened with a Schlegel diagram projected on the facet
  defined by the selected vertices.